They kill credibility like nothing else can.
If you choose to market yourself (it can be done--George Lakoff's "Don't Think of an Elephant" comes to mind, he eventually gained a publisher when word-of-mouth got people interested).
Here are a couple of things to avoid from a library buyer:
If you offer a book to a library, do not expect payment or a review of it. It is considered a gift by postal regulations, and the library is then free to use as it sees fit. We simply NEVER offer reviews; most public libraries haven't got anywhere near the staff to manage this once-common practice anymore.
DON'T SHRINK-WRAP! Can you tell I mean this?! Just, please, don't do it--I don't care what the book is.
NEVER, NEVER expect to meet with a purchasing executive unannounced. There are tools at your public libraries (Bowker guides--the librarians will be very familiar with them) to let you know who makes collection development (read that as "buying") decisions. Make an appointment. CD librarians are incredibly busy, and you don't want to leave a bad impression with people who can be of possibly tremendous service (my boss, for instance).
Don't spam-review Amazon, it isn't considered a reliable source for reviews in the profession; try your best to get a review in a local newspaper or industry source.
You can also offer a quality bound item to Ingram and Baker and Taylor directly; they are very savvy about independent works that show promise. This is a tremendous help in the credibility game!
Try a publisher affiliated with this group:
http://www.ipgbook.com/They know the ins and outs of marketing very small presses and have respect in the biz. They are very fair to authors and VERY professional.
Good luck!