The Elements of Typographic Style
BY ROBERT BRINGHURST

If you buy only one book on type, this should be it. Put it right next to your Strunk and White. We particularly liked what this Amazon user said: "Bringhurst puts the why before the how. His work is a mixture of a history, discussion and practical advice. Bringhurst values timeless typographical virtues higher than the fashions of the day." And how. He's the anti-David Carson.

Stop Stealing Sheep & Find Out How Type Works
BY ERIK SPIEKERMANN, E.M. GINGER

An excellent text for the beginning designer or layman. It's accessible, loaded with shrewd examples and not nearly as academic or poetic as the Bringhurst book. In the same ilk, you might try Robin William's The Non-Designer's Type Book, or any of her Not A Typewriter books.

The New Typography: A Handbook for Modern Designers
BY JAN TSCHICHOLD

This is a reprint/translation of a the great German typographer's 1928 text. Not for the faint of heart. Truly a typophile's book - but there is still some quality practical knowledge inside.