The Complete Guide to Self-Publishing by Tom and Marilyn Ross was my self-publishing bible. There is one other book that I wish I had known about at the time and that is The Shortest Distance Between You and a Published Book by Susan Page.
The Internet also offers lots of information on self-publishing, but, for beginning self-publishers, I still recommend a trip to the local library or your favorite bookstore. Here's why: there are many companies preying on the would-be self-publishers lack of experience. Their websites offer some good information, but many are simply well-disguised sales pitches for their goods and services. That's not to say that all companies offering self-publisher support services are bad. Rather, if you know what's involved in publishing your own book, you'll be able to differentiate between necessary expenses and what I like to call Flim-Flam.
http://www.suite101.com/welcome.cfm/self_publishing
This is a very complete site with articles on editing your work, finding a reputable printer, finding a distributor/wholesaler, and much more. Also includes a good discussion forum for beginning self-publishers.
http://www.bookzonepro.com/
This site offers "tips and tools for publishing professionals" and is geared for more knowledgeable publishers. Be sure to check out their resource section too. (Prepare to be overwhelmed...but in a good way.)
http://www.spawn.org/selfpub/selfpub.htm
Small Publishers, Artists & Writers Network (SPAWN) was formed to "provide education, information, resources and a supportive networking environment for creative individuals and small business owners interested in the publishing process."
http://www.lcweb.loc.gov/loc/infopub/publish.html
Library of Congress
http://www.bowker.com/standards/home/isbn/us/printable/abi.html
R.R. Bowker
http://www.pma-online.com/
Publishers Marketing Association