I have three books on writing that I turn to every now and again because they are that good! In no particular order, they are:-
1. Self-Editing For Ficton Writers by Rennie Browne and Dave King. This is a great book for helping me to whip my manuscripts into shape. For possibly any number of reasons, my default tense appears to be the passive (see what I mean) and this book keeps me aware that I need to keep my writing fresh and vivid, not deaden it with dead wood. If you want help tightening up your writing, keeping your dialogue realistic, and making your characters come alive - this is the book for you.
2. Writing Fiction: A Guide to Narrative Craft by Janet Burroway. This was one of the textbooks for a college class and I still have it. Burroway also covers chracterization and the importance of showing over telling but she goes into greater depth than Browne and King and also uses examples from authors as diverse as Jamaica Kincaid and John Cheever to make her points. If you're having trouble actually thinking of a subject you actually want to write about or developing your themes, time spent with this book will help.
3. On Becoming a Novelist by John Gardner. Gardner wrote 25 books and taught writing at various universities and this book is the distillation of his experiences as a writer. If you dream of being a writer, read this. He talks about honing new ways of seeing the world, abouit the hallmarks of good writing, how to balance work and writing, the disappointment of rejection and the pitfalls of success.