Woe Is I: The Grammarphobe's Guide to Better English in Plain English, Patricia T. O'Conner,
Informative, funny, a great book about the mechanics of writing. Her coverage of punctuation should be required reading for all would-be book authors. Fairly inexpensive, even (eleven dollars US, list). Highly recommended.
I haven't read her other book, Words Fail Me, ISBN: 0156010879 ; anyone? -- PaulChisholm