The Elements of Style, by Strunk & White

This is the quintessential English grammar book, a handbook for the care and feeding of the English language. Originally written by a Cornell professor, it was later revised by E.B. White (of Charlotte's Web and the New Yorker fame). The most recent incarnation is a lovely red hard-bound version with whimsical illustrations.

The Elements of Style has been a beacon of sorts to writers for decades. The chapters cover topics such as punctuation, spelling, commonly confused words, and development of style and structure. Imperatives are followed by examples of use and misuse. The discussion is intelligent and wry, with statements such as the following:

Colloquialisms. If you use a colloquialism or a slang word or phrase, simply use it; do not draw attention to it by enclosing it in quotation markes. To do so is to put on airs, as though you were inviting the reader to join you in a select society of those who know better.

Within the chapters are nuggets of wisdom that most people manage to forget after elementary school, and are vital for anyone hoping to pursue writing in any capacity, from professional emails to full-time staff writing. Strunk and White continually drive home the need for concise writing. The slim book ends with a glossary of grammatical phrases, and the illustrated edition includes portraits of the authors.

If you consider yourself a writer, this is the best little reference I've seen. If you are annoyed and dismayed by today's continual butchering of English, you'll love this book. If you don't know of any such butchering, you need this book.