The Authors Guild
The best bread-and-butter group for professional writers, and a great source of usable news as well as member benefits (legal advice, insurance, and more). If you're eligible, you'll probably want to join.
Book Industry Study Group
BISG works in many ways to improve interaction among publishers, authors, booksellers, librarians and others. Publishers (including self-publishers) should be sure to explore the site for leads to valuable events, publications, and tools. Publishers who become members have a chance to influence the way the publishing industry works by participating in BISG committees.
IBPA, the Independent Book Publishers Association
Open to self-publishers and other writers as well as to independent publishers of all sizes, the Independent Book Publishers Association formerly known as PMA provides a cornucopia of marketing opportunities and how-to advice. Members can access articles from the IBPA Independent by topic and author online.
For those who'd like to hire out their writing talents ñ by the hour, by the project or by getting a job. Check the ever-changing listings from publishers and others.
Lots of job listings, which you can access without paying, plus blogs and a wide range of informative material.
Poets & Writers
Look here for a wide variety of information about resources for writers ñ especially but by no means only those who write fiction and poetry. And join to receive the group's monthly magazine, which can help you get publishers, readers and funding.
Michael Cader's email newsletter takes you inside book publishing; his news analysis is always insightful, and the databases he has created can tell you ñ among other things -- which editors and agents are involved with which books.