I use The Oxford Dictionary of English most of the time (the one with the British spellings), although I have a good old Webster's Dictionary around here too. But any good dictionary will help you with definitions and spellings and, for those special occasions, with word origins and date of first usage. (In case you're wondering whether your heroine from 1750 would use the word "cool" to describe something that's spectacular or interesting. And the answer is NO.)
A good thesaurus
Yeah, I know there's one built-in to whatever word processor you're using, but it's not as thorough as a really good one; in fact, it really only operates like a smart dictionary. I have a huge Bartlett's Roget's Thesaurus from Little, Brown. It took a while for me to really, truly learn how to use it properly, but it has resulted in some startlingly better word choices for me from time to time.
The Elements of Style
What do you mean, you hate that book? You're just saying that because visions of English 101 still haunt you. Don't just buy it: READ IT! My personal favorite rule is #17:" Omit needless words", beautiful not only for its advice but for the way form meets function. However, there's lots of good stuff here. Practical advice on not overwriting, plus helpful info for folks who can't remember how to form possessives properly, or when to use certain pronouns (its and it's, say).
Perhaps my favorite of all writing books, offering inspiration, encouragement, honesty, humor and practical tips. Since I read this one often, I'm glad I bought the hardcover, but at least spring for the trade paperback!
Bird by Bird: Some Instructions on Writing and Life
This book offers inspiration and encouragement for writers, no matter what part of the journey you may be in.
Still Writing: The Perils and Pleasures of a Creative Life
by Dani Shapiro
More inspiration and encouragement for writers, along with the occasional bit of concrete writing advice.
The Creative Habit: Learn It and Use It for Life
This extended work on creativity by esteemed choreographer Twyla Tharp contains gobs of advice that applies equally well to a writing life.
It's a woman's prerogative to include books in more than one category. Stephen King's book offers practical tips about writing and submissions, and who better than an author who has sold a bajillion books to give advice?
You Can Write Children's Books
Also, there's the You Can Write Children's Books Workbook from the same author. Both offer concrete, specific advice to get you started in the world of children's books. I'm not sure I totally understand the benefit of having both of them, but it could be that the Workbook came too late for me in my personal process.
Picture Writing: A New Approach to Writing for Kids and Teens
One of the best resources on the market for writing for young people. Also, one of the most challenging: it requires discipline and effort and time. But Anastasia is an excellent teacher: if you follow the program, you'll end up with decent work product. Something you can feel comfortable showing to other people for feedback.
Writing Picture Books: A Hands-On Guide from Story Creation to Publication
by Ann Whitford Paul
A veteran in the picture-book field provides practical writing tips for every step of the process from idea conception to finished manuscript.
How to Write Attention-Grabbing Query & Cover Letters
Terrific resource for folks who are ready to send their manuscripts or magazine articles out into the world. Provides how-to (and, for illustration purposes, how NOT to) tips for preparing query and cover letters.
Self-Editing for Fiction Writers, Second Edition: How to Edit Yourself Into Print
Excellent resource on revising and refining your work to get it ready to send to editors and/or agents. The one book on revision to have, if you're just having one.
I love this book. Bugeja gives practical advice on developing ideas for poems, then provides a tutorial on the components of poetry: voice, meter, and rhyme, with attention to the title, lines, and stanza forms, followed by an explanation and study of the various forms and formats poems take. The one book to have if you're having only one.
A Kick in the Head: An Everyday Guide to Poetic Forms
Technically not a how-to guide, this picture book provides examples of 29 poetic forms, with basic definitions and formats explained. For visual thinkers like myself, this one's a must.
The Ode Less Travelled: Unlocking the Poet Within
by Stephen Fry
A terrific resource for learning to understand and write form poetry, from renowned actor and poet, Stephen Fry.