Oh, you bespectacled, bookish, innocent old man. You harmless, selfless, peace-loving soul! How much you have been made to suffer in your Arkham career! It makes me shiver to think of it. The investigators use you, and then they lose you. They send you into the stacks for that all-important tome that only YOU know how to find. Then they whisk it from your hands and, by way of thank you, kick you, screaming, into the jaws of the next ghoul who wanders by. You serve the investigators, and then the investigators serve you (to the ghoul, that is).
I cannot save you from your fate, you bow-backed bringer of books, you ambler of aisles, you sorter of scrolls and meal of monsters. But I can honor you by providing an updated review of what you add to our decks, all while receiving nothing -- oh, worse than nothing! -- in return.
LikeAssur's review below was right for the moment (three years ago), but since then much has changed, and the humble research librarian has become an increasingly excellent asset. Here are the main changes:
1) We now have quite a few very high-powered tomes. Pnakotic Manuscripts and The Necronomicon occupy the top shelf of the updated tome library. The new Old Book of Lore is also quite good. These tomes cost quite a bit of XP, so it's be a shame for them to molder at the bottom of your deck all scenario. Research Librarian prevents that from happening. Think of it this way. You know that granddaddy of all search cards, No Stone Unturned. It costs a gigantic 5xp. If the best couple cards in your deck happen to be tomes, then in many situations the research librarian is just as good, at 0xp! If you don't have an ally out, he's actually better, crazy as that sounds. In the past he may have only been able to find you, at best, the Encyclopedia. But our man ain't dealing that weak sh*t anymore. Like that kid in Breaking Bad, he's got the REAL stuff now. It ain't always blue, but it IS always magic.
2) There is an easy, SO easy way to offset the Research Librarian's 2 resource cost. Just stick a copy or two of Astounding Revelation in your deck. Since the RL searches your whole deck, Astounding Revelation is guaranteed to trigger every time you play him. I do hesitate to recommend this strategy on humanitarian grounds, though... I really don't know how many mind-blowing revelations this poor guy can take. It just seems mean, every time you send him for a tome, to also demolish all his safe assumptions and confront him with the terrifying magnitude of the mythos.
3) The Research Librarian can now be recycled. If you have him in your deck, also consider a copy or two of Calling in Favors. This card lets you pull the librarian back to your hand, and go fishing in your deck for a stronger ally, offsetting the cost of the new ally by two. Thus, the RL not only lends you sweet books for free that you, because you're a crappy friend, never return; he'll also introduce you to his cool best friend Dr. Milan, who, because you are a REALLY crappy friend, you'll start hanging out with separately.
There are two takeaways here. 1) The Research Librarian is a giving, lovely person who makes our decks better in so many ways. 2) We do not deserve him! The RL deserves an investigator willing to invest in a committed, long term relationship. I will be that investigator! When I get around to it, I'm going to post my Literary Lives Matter deck to Arkhamdb, which will have the research librarian as the only ally, NO tomes because, seriously, he's an ally, not an errand boy, and two copies of Trusted, so my man knows he's appreciated.