Elements of Arousal: How to Write and Sell Gay Men's Erotica by Lars Eighner. Excellent resource for an erotica writer who's looking to make some serious improvements in their work. Not as specific as the title suggests, though the marketing tips are now out of date.
Self-Editing for Fiction Writers: How to Edit Yourself into Print by Renni Browne and Dave King. Excellent resource for learning how and when to edit yourself. I recommend rereading thios one every few years.
The Writer's Guide to Queries, Pitches and Proposals by Moira Allen. Solid overview, including examples, of how to create the aforementioned queries, pitches and proposals.
Booklife: Strategies and Tips for the 21st-Century Writer by Jeff VanderMeer. This is the advanced learning class for the more established writer: a resource on how to build a career and keep it going by a writer who knows.
Creating an Online Presence: Career Building for Writers by Cat Rambo. Excellent resource for the writer who needs to get more familiar with how to build themselves up online.
A short list of (some) books on writing erotica/erotic romance:
How to Write and Sell Erotica by M. Christian
How to Write Erotic Fiction and Sex Scenes by Ashley Lister
How to Write Hot Sex: Tips from Multi-Published Erotic Romance Writers
Erotica Readers and Writers Association - blogs, reviews, etc.- http://www.erotica-readers.com
What are some of your favorite writing resources and books?