Well the Space Marine codex can use Pyromancy, Biomancy, Telepathy and Telekinesis.
You play a fire base force, imperial fists specifically.
You can either take Biomancy powers, to give your Librarian a bit more close combat effectiveness or squad buffs. Iron Arm and Warp Speed are both good, as it makes your Librarian that much more effective in close combat. Enfeeble is fun, making an enemy squad weaker for your shooting. And if you can get it, Endurance is Got-Dang awesome. Slap that on any tactical or devastator squad and it just gets interesting. (Feel no Pain, It Will Not Die, AND Relentless). Also Haemorrhage. Short range but REALLY fun power.
Telepathy has some fun stuff in it, I particularly like Invisibility and Terrify. Puppet Master is another I like a lot. "Oh.. you just shot your own wave serpent with a D-gun from the rear? I'm sorry.." And Psychic Shriek is always fun too, but again, like Haemorrhage, short range.
Telekenesis is one of the Disciplines I would tend to avoid more often than the above two if it weren't for one power. Objuration Mechanicum. Having a power than auto-hit's flyer vehicles out to 24" with the Haywire special rule. I made an I.G. player quite upset when my Librarian dropped a Penetrating hit with the Haywire roll and took out his Vendetta on the same turn it came in. Also, Gate of Infinity and Crush are fun too, if not random. I'd say, outside of the Objuration Mechanicum power, this is the weakest of the disciplines. Still, vs. a vehicle heavy opponent, might be worth taking a roll and hoping for Objuration.
Last but not least, Pyromancy. Again, I want to like this discipline more, but a lot of the powers seem kinda 'meh' to me. Fire Shield and Firey Form are probably my two favorite powers on the list. Both are pretty neat I think. Firey Form being my favorite.
Overall, here's what I would do. I'd try and learn all the powers from the above disciplines. Or at least, know some of the better/best powers in each of them and which discipline they belong to. Then, look at your opponent's force and figure out what the best discipline will be to deal with their army. Once you've done that. Hope for good rolls on the chart or charts (depending on if you choose two different disciplines).
Best of luck and remember, don't be afraid to use your Librarian as a utility character based on what powers he gets. It's random, so if you don't get what you want, at least try and put him somewhere in your force where his powers will be useful to the squad/unit he's near or attached to.
Take it easy.
-Red__Thirst-