Yes, I know what you're thinking -- more books to read? Well, think about it this way: it's an investment into your writing and yourself. And, what's more, you don't need to buy these books (that's what libraries are for!). Once you get them, just read them a little at a time (a chapter every evening, for example) or skim them for the most helpful parts. Don't be overwhelmed; check out the links and bookmark them/write them on your library list for future reference!
Now, without further ado, my writing resource list:
Techniques of the Selling Writer, by Dwight Swain (I also have a blog post on it here.)
Writing Fiction for Dummies, by Randall Ingermannson
Live Writing, How Writers Work, and A Writer's Notebook, by Ralph Fletcher (They're written for young writers, but still have plenty of wisdom for the rest of us.)
The Elements of Style, by Strunk and White
Plot & Structure, by James Scott Bell
Stein on Writing, by Sol Stein
Self-Editing for Fiction Writers, by Renni Brown and Dave King
Outlining Your Novel, by K. M. Weiland (Even if you don't enjoy outlining, read this book, and you may be surprised.)
Getting into Character, by Brandylin Collins
The Art of Fiction, by John Gardner
Story, by Robert McKee (Essential for all writers of screenplay, and good for the rest of us too.)
Writing the Breakout Novel, by Donald Maass
Writing Magic: Creating Stories that Fly, by Gail Carson Levine
Seize the Story, by Victoria Hanley
For fantasy writers in particular, I highly recommend reading J.R.R. Tolkien's essay "On Faerie Stories" (from Tolkien's Tales from the Perilous Realm). I also recommend Orson Scott Card's How to Write Science Fiction and Fantasy.
For poetry writers (yes! at last!), I absolutely love Mary Oliver's A Poetry Handbook and Ted Kooser's The Poetry Home Repair Manual. Both are invaluable to any aspiring poet.
Now it's your turn: what books on writing and/or writing resources have you found to be most helpful?