The Elements of Style, William Strunk, Jr., and E. B. White
How to Write Powerful Metaphors (This blog entry is the best quick guide to metaphor I've seen.)
Literary Devices, Terms, and Elements
Steering the Craft: A 21st-Century Guide to Sailing the Sea of Story, Ursula K. Le Guin
TimeSlips.org (This site was designed to provide creative interaction opportunities for elderly people in assisted living and other caregiving situations, but it's a great source of writing prompts. Use the images themselves as prompts, or let the collection of entertaining stories told by older people inspire you.)