My new favorite reference book:
The Oxford American Writer's Thesaurus, 2nd ed., currently selling for less than $25 at Amazon.com—and many other places, I'm sure. For all those times when a word dangles just beyond the edges of your brain or you realize you want something more evocative than "interesting" or "important." And because many of the contributors are practicing writers, the book devotes a fair amount of space to context and nuance (for example, the difference between fiction, figment, and fabrication).
Not quite as good, but useful if you have trouble remembering the difference between, say, affect and effect, is Dave Dowling,
The Wrong Word Dictionary: a list of 2,000 commonly misused words. Although Amazon.com lists it as shipping in 1-3 months, if you go to the list of alternate suppliers, you can get it right away for $6 including shipping and handling, less than Amazon.com is charging. You know if you need it.
The best writing book I've read recently is Laurie Alberts,
Showing and Telling, on the uses and benefits of scene and summary and how to blend them in your fiction. It's a nice corrective to the "show, don't tell" mantra, even as it supports the basic point behind the mantra.
And while we're on the subject of books, I am also reading
The Social Animal, by David Brooks (really interesting discussion of psychology, cultural influences, and more) and Alison Sim's
The Tudor Housewife—admittedly not for most people but invaluable for those of us with characters inhabiting the 16th century, and not only in Tudor England. It's beautifully written and blessedly short, suitable for perusal in the evenings after a day's work.
Best,
M