Self-Editing for Fiction Writers by Renni Browne and Dave King. I have used this book to supplement writing advice in textbooks for courses that I have taught. Browne and King talk about the evils of trailing participial phrases and cliched writing and about the goodness of proper dialogue formatting and concision. Its exercises are useful because they are practical and have suggested solutions.
Word Menu by Stephen Glazier. Glazier's book is a thesaurus-style dictionary that is organized not by word so much as by discursive category. For example, I have always been fascinated by the terminology of religious clothing. I can look at the liturgical vestments section and find out what an amice or tunicle is. Then, if I ever have to describe a priest getting ready for mass, I can use these words. Each word entry has a definition (an amice is a "priest's square cloth worn over the shoulders"). Answers.com seems to have bought the rights to this book, so you can do electronic lookups. (I have only ever used the print version, but that may change.)
Writing Down the Bones by Natalie Goldberg. It is organized into short chapters that explain how Goldberg goes about her writing life. It offers practical advice but is also, in part, a member of what I like to call the emotional support category of books about writing. For example, it has chapters entitled "Original Detail" and "The Action of a Sentence" but also "Don't Use Writing to Get Love" and "Doubt Is Torture."
Understanding English Grammar by Martha Kolln and Robert Funk. Yes, grammar is important. Writers need to know the rules so that breaking them is an act of choice rather than an act of ignorance. For example, to understand the evils of trailing participial phrases, one must know what a participial phrase is. To find out, you can look up "participial phrase" in Kolln and Funk's index. This book uses some linguistic terminology, so the book has educational value as well. On that note, this book's exercises have answers.
If You Want to Write: A Book about Art, Independence and Spirit by Brenda Ueland is a full-fledged example of the emotional support category of books about writing. It gently explains why writing is the best thing a writer can do. Unlike many people in one's life, most likely, Ueland says that the creative act is important. She discusses William Blake and Vincent Van Gogh, and rather than describe them as crazy people (which noncreative people like to do), she lauds them as creative geniuses who loved imagination and treated art as the highest form of human participation in the world. Chapter titles include "Everybody Is Talented, Original and Has Something Important to Say" and "Why Women Who Do Too Much Housework Should Neglect It for Their Writing." Isn't that nice to hear for a change?