The Lively Art of Writing by Lucile Vaughan Payne. One of the clearest and best writing books. A bit dated now, but worth reading. She practices what she preaches.
The Elements of Style by William Strunk, Jr. and E.B. White. A little classic on writing well. Also, a bit dated, but loaded with good advice.
The War of Art by Steven Pressfield. Inspirational and stirring. "DO THE WORK!" When I feel I can't write, I read a few pages of this book.
Plot by Ansen Dibell. Covers exactly what you'd expect from the title and does it well.
Roget's College Thesaurus and The New Merriam-Webster Thesaurus. I know there are online thesauri by the dozens, but I prefer these two. They give me enough choices without overwhelming me.
Baby Names From Around the World by Maxine Fields. Helps me name my characters. Invaluable when I'm looking for the perfect name for a character.
The New Merriam-Webster Dictionary and The American Heritage Dictionary. Once again, although there are many dictionaries online, these do right by me.
The Random House Handbook and The St. Martin's Handbook. These can answer any grammar and punctuation question I have. I like both, but Random House seems better organized.
Barlett's Familiar Quotations by John Bartlett. I never trust online quotes.
Those are the books I reach for time after time. What writing books are on your shelves?