Special Characters aside, the Space Marine Librarian is one of your 5 basic options for an HQ selection (1 of 4 if you count Captain and Chapter Master as basically the same). At 100 points, he is priced exactly the same as the SM Captain, the SM Chaplain, and the Master of The Forge. These are the main options we will be comparing the Librarian to.
WS-5, BS-4, S-4, T-4, W-2, I-4, A-2, Ld-10, Sv-3+
Looking at the stats alone, the Librarian and the Chaplain are evenly matched. The Captain appears better suited for close combat with a higher WS, W, I and A value. The Master of The Forge looks better suited for long range with a lower WS, and a higher BS than the Librarian. We're seeing some early role-defining already.
The SM Captain's Wargear Options only further his role-definition as close range. This is not to say you could not build him to be a mid-range, but that his options allow him better utility in close range than the other HQ options.
Conversely, the Master of The Forge's options only further his role-definition as mid-long range. Taking the Conversion Beamer especially will make him long range, as you will want to be further than 4 feet from your target to maximize that gun's power.
Now, down to the Librarian and the Chaplain. The Chaplain's equipment and options seem to direct him towards a close-mid range offensive support. He's geared for close combat, and bestows any squad he's attached to both Fearless and Liturgies of Battle, which allows them re-rolls to hit in their first round of an assault.
Librarians appear to be the most versatile of the options. This is mainly due to their large selection of Psychic Powers. Their uses and abilities vary so drastically from each other, that you could find your Librarian filling any number of roles on the battlefield depending on which two you choose.
Your best option here if considering taking a Librarian is to build out the rest of your list first, then decide what role you want him to take, and equip him accordingly.
Force Weapon: This is a Power Weapon with an extra Psychic Power built in. If you inflict an unsaved wound on any multi-wound creature in close combat, you can attempt to inflict Instant Death. This requires a Psychic Test, as a normal power, and counts against your limit of powers/turn. This is a great tool that will allow your Librarian to help out in assaults, especially against other characters.
Psychic Hood: This tool allows your Librarian to fill a very specific defensive role against enemy Psykers. If any enemy model within 24" of your Librarian passes a Psychic Test, you can declare that you use your Psychic Hood. This is a roll-off between your Librarian and the enemy Psyker. D6+Ld. If you beat your opponent's score, the power is nullified and does not take effect. Fun to note: The Psychic Hood can be used once each time an enemy model uses a psychic power within range. Multiple Eldar Farseers? You can attempt to nullify EACH POWER one by one.
Psychic Powers: You get to choose 2 powers from a list of 9. For free. They do not cost you anything to take. I'll go over each power in a bit.
Epistolary: Normally a Librarian can only use one psychic power per turn. Upgrading to an Epistolary (+50 points) will allow you to use two psychic powers per turn. 50 points is a hefty price to pay, and keep in mind that you can only use one psychic "shooting attack" per turn, so to really maximize this, at least one of the powers is going to have to be a support/utility power. My advice to you is to decide what to take for the rest of the army, then decide what role needs filling/bolstering. Take a Librarian and choose a power to help him fulfill that role. Lastly, decide if you really need a second power. Will it help him in the role you need him to fill? Will it allow him some versatility that you feel is necessary?
You can replace his boltgun with a Storm Bolter(+3 points), Plasma Pistol(+15 points), or a Combi-Weapon(+15 points). Also important to note: You cannot both fire a ranged weapon and use a Psychic "shooting attack" in the same turn, so make this choice wisely.
Terminator Armour: Depending on what else you give him, the cost varies. If you are planning on taking a psychic "shooting attack," and think you'll be using that instead of any gun, you can take Termie Armour with no gun for only 25 points. This is the cheapest that any basic HQ can take Terminator Armour. You can also take Terminator Armour with a Storm Bolter(30 points), a Combi-Weapon(35 points) or with a Storm Shield(40 points). Be careful taking this route as your HQ is getting mighty expensive.
He may alternatively take a Jump Pack(25 points), or a Space Marine Bike(35 points). Mostly, I would suggest taking one of these options only to allow your Librarian to better attach to an Assault Squad or Bike Squad, though there are some other more interesting opportunities that lie with taking one of these options...
There are 4 "shooting attack" powers and 5 support/utility powers. Let's cover the "shooting attack" powers first.
Range 12", S-4, AP-2, Assault 4. At AP-2, you're looking at a great anti-Elites power. Assault 4 can really do some devastation to an opponent's heavy-armored infantry. Also at 12" Range, any unit he's attached to can really let rip with Rapid Fire weapons. I'm thinking specifically of Sternguard Veterans here, as they can really help to maximize this role. Consider taking Epistolary and a second power of either Gate of Infinity, Null Zone, or Force Dome depending on if you think this unit would benefit most from better mobility, possibly denying inv. saves, or survivability, respectively. Another possibility is to take a Jump Pack and attach him to an Assault Squad or Vanguard Vets. Smite would serve to soften up your target pre-assault.
One useful role I've used with my Librarian in the past is take him as an Epistolary attached to a Tactical squad. Gate of Infinity gets them close to the Enemy, and then Smite and normal shooting attacks will help cut through enemy troops. (Wanderer787 of Dawn Crusader Chronicles)
Machine Curse is a 24" Range power that only affects Vehicles. While useful if you are expecting a mechanized force, this power seems a little underpowered compared to the others simply because it only causes glancing hits (it does cause them automatically, though, regardless of the Armour Value).
Template weapon, S-5, AP-3, Assault-1. This is another good power to give a Librarian attached to an Assault Squad or a Vanguard Vet Squad, given the range. Template weapon gives the added bonus of ignoring Cover Saves, so this is a really choice pre-assault power.
Vortex of Doom
Range 12", S-10, AP-1, Heavy-1, Blast. This is extremely powerful, and also slightly unreliable. Note that a failed Psychic Test will result in the blast marker being placed ON THE LIBRARIAN, with no scatter. This unreliability makes it somewhat less beneficial to attach him to another unit. A Vortex of Doom Librarian seems better off solo.
The short Range and Heavy-1 also make this power very difficult to fire off effectively as an infantry unit. This is where an interesting choice of wargear can come into play. Both Terminator Armour and Space Marine Bikes can fire Heavy weapons even if they moved during the movement phase.
Given that you might be taking a Librarian with this power solo, giving him Terminator Armour isn't such a bad idea. He can stomp around mid-field and fire off his Vortex of Doom as many times as possible until your opponent eventually takes him down (and your opponent WILL be gunning for him, especially if solo). If you're the uncaring type of commander, you can attach him to a Combat Squad, giving him some meat-shields to allow him another round or two.
A Space Marine Bike offers another interesting combination. This Librarian seems more of a suicide-bomber. Run him deep into enemy territory and terrorize your opponent, affording the rest of your army less resistance in doing what they need to do. Make sure your marines have wished their Battle-Brother well before he goes roaring off into the sunset, as he'll most likely not return. This ability tends to make your opponent focus-fire with extreme prejudice.
Now, on to the Support/Utility powers...
For an entire turn, your Librarian and any unit he is attached to will receive a 5+ invulnerable save. This is great for adding just a little bit of survivability to a unit which will most likely attract heavy fire in an attempt to neutralize your Librarian. In addition to adding survivability to a possible assault-role, this could also do well attached to a Troops selection, providing assistance in holding an Objective.
Gives your Librarian the Fleet special rule and an Initiative of 10 for the duration of that Assault Phase. Attached to an Assault-oriented unit, this would help out immensely in such a role, especially against Elite units or other high-Initiative opponents. If taking this route, consider upgrading to an Epistolary and taking Might of The Ancients as your second power.
Might of The Ancients
This gives you Strength 6 (and 2D6 for armour pen) for the remainder of that Assault Phase. Lemme think here... An Epistolary with this and Quickening (assuming standard equipment, and that you chose Bolt Pistol) would be doing 3 attacks (4 on the charge) at Initiative 10, with a Strength 6 Power Weapon. Huh.
The Gate of Infinity
You can warp anywhere within 24" as if you just came in via Deep Strike. This ability does extend to any unit your Librarian is attached to, with the added possibility of you automatically losing one model as a casualty if you roll doubles on a scatter roll. Given that this warping will deny you the ability to assault (acts like Deep Strike), if you take a unit with you, it should probably be a shooty one, like Sternguard, Terminators, Legion of the Damned, or even just a Tactical Squad. Especially useful if you do attach to a Tactical Squad to attempt a Turn 5 jump to an Objective that is un-claimed, or attached to anything in a Turn 5 jump to contest a claimed Objective.
An interesting thing to try with Gate of Infinity is to have several drop pods with locator beacons in your army, which allow you to deep strike unerringly if you're close enough to the pod.
Doing it that way lets you jump around from place-to-place without any worry that you'll randomly deep strike in the wrong area. If you combine it with other deep striking stuff, like termies, you could have an interesting setup for an army.(Pcrackenhead)
All enemy units within 24" of the Librarian must re-roll successful invulnerable saves for the remainder of the player turn. If used in conjunction with another ability or an anti-Elite role, this could do some serious damage. I can think of a few units that would really have a hard time with this (Farseer, Warlocks, anything from the Daemons army; seriously, the inv saves are all they have!).
Chief Librarian Varro Tigurius
Tigurius is our Libriarian Special Character. At 230 points, he's quite the investment, so make sure you really want him before you take him. Let's see what you get for all those points:
Master Psyker: He has ALL the above listed spells. All of them.
Gift of Prescience: You can re-roll any Reserves rolls, even successful ones.
Hood of Hellfire: It doubles as both a normal Psychic Hood and allows him to use THREE psychic powers per turn.
Rod of Tigurius: A master-crafted Force Weapon, allowing him re-rolls to attack.
... and that's it. In all other respects, he is just like a normal Librarian. (...now that I think about it, that's kind of like saying "Other than his god-like powers, he's just a normal human." Silly of me.)
My purpose with these reviews is to simply provide all you 40K enthusiasts out there with a collection of thoughts and uses for each unit; i.e. how is it useful, and in what way? I do not claim to know everything or be perfect in any way, so if you feel that I have made a mistake or overlooked some important factor in any of these reviews please feel free to leave a comment. If I agree that it is important or you point out that I have made a mistake, I will adjust the post and give credit to the poster that prompted the edit.