The Publishing Room
Maintained by Raymond Long. Disclaimer: any information on this website could be wrong.

Most of the websites listed here I haven't investigated thoroughly.

Goodreads - self-described as "a large library that you can wander through and see everyone's bookshelves, their reviews, and their ratings" - horror website - self-described as "Horror's #1 community on the web" - British SF website - authors can publicise their media events
SFF.NET - Many resources.
Author showcases - authors can put their work online
Groups authors can join
Writers Guild of Great Britain
Society of Authors (UK)
Australian Society of Authors
New Zealand Society of Authors
The Authors' Register
Listings of publishers, agents, marketing companies, etc.
WritersServices - US agents from Writers' & Artists' Yearbook.
WritersServices - UK agents from Writers' & Artists' Yearbook.
Association of Authors' Agents (UK) - Lists names of agencies only, not contact details.
Australian Literary Agentsí Association
Piers Anthony's Internet Publishing
Literary Market Place
Industry codes of practice, standard rates of pay, etc.
Writers Guild of Great Britain - Lots of documents available for download.
Australian Society of Authors - Recommended rates of pay in the Australian market.
Advice on writing, getting published, marketing books
Miss Snark, the Literary Agent - A literary agent's blog, setting out why books get rejected.
The Reason Editors Reject Manuscripts - Survey of reasons for rejection.
How to market your book on the internet
Writers and Artists (UK)
Ebook Crossroads
Writers weekly - ezine
Written Road - travel writing
Book Promotion Newsletter - ezine
Writers Write
Wheatmark - download free book marketing guide
Foner Books - marketing article dated 2009
Wikipedia category: book promotion
Book promotion/PR companies
Pump Up Your Book Promotion
Book Promotion 101
Smith Publicity
Advice on marketing in general, not books specifically - free book on marketing