In 1993, I joined Publisher's Marketing Association. From the first day, they offered great advice, but mostly they offered me a chance to be with like-minded people at events, and to talk to people about what they did for their own books, from printing to marketing. Truthfully, the organization changed the direction of my career and I fell in love with the independent publisher/author/illustrator/expert members in the group and elsewhere in the world.
In 2009, they became the Independent Book Publisher's Association, focusing more of their resources and attention on the business aspects of book publishing. It's been a valuable relationship, and I have met some amazingly creative people. If you plan on publishing a book, I highly suggest joining IBPA, one of their affiliates, or another association. Don't just join. Participate and be involved for the greatest payback.
What other organizations do you rely on for your book publishing and marketing?