I send this letter out to writers everywhere. (Since I'm one of you, I feel I can do this.) Librarians are your friends. Librarians will let you (politely) ask them to add your book to their collection. Librarians will work with you to set up new-author programming, a book signing or reading, and ten million other things that can help you out when your book is born. Heck, librarians will even read your book and write reviews, both on their blogs and in the trade pubs librarians use to make purchasing decisions.
Librarians are very well connected - it's a microverse, really, and the librarians who don't know each other on Twitter, or Friendfeed, in person or by reputation is a pretty small number.
While we don't require that you bribe us - we're an ethical lot, really (we even have a Cod of Ethics) - most of us will try extra hard to help you if a rogue brownie or cookie (or cup of coffee) makes its way to our desk. We're easily kept contacts - we're online at all hours and up on the latest fun tech. We're plugged in, and most of us - or those of us who control the library's web content as well as our own web presence - have a guaranteed audience.
Given all of this, please do not annoy your librarian. Do not lie about who you are (we can hunt you down by ISP if you're sloppy abou tit), troll the comments in our blogs, make outrageous accusations, and especially don't threaten librarians. Please see the strange case of Sergio Rivera-Ayala as told by our dear Pegasus Librarian. Not only does all of this look ridiculous, but I now know a TON of librarians both domestically and internationally who wouldn't touch this book with a fifteen foot pole. Very unfortunate for both the writer and his publisher, who I'm sure would prefer this not have happened.
Be ye both encouraged and warned. Librarians travel in packs, are happy to help, and we're notorious gossips. If you show your behind, good or bad, be assured we'll all be chatting about it.