By William Strunk Jr. and E.B. White
I have used this book faithfully since high school. Nowadays, of course, there are computer programs that have replaced most grammar and punctuation books, but what computers haven't replaced, and what this book also seeks to address, is the question of style.
I, like many, initially chafed at Strunk and White's ideas. Once I accepted that you had to learn the rules before you could break them, I set about what would become the oddly comforting task of getting to understand the true and ordered nature of the universe of writing. Perhaps the best thing in the book was what it taught (or tried to teach) me not to do. Their caution against overwriting, overstating, injecting opinions, and going on at length (I'm in serious danger of it here, in fact) was sorely needed.
This book is an essential tool. It has been of great use to me and is probably responsible for my best writing. I owe my successes to Strunk and White; only the mistakes are mine.