All You Need to Know About the Music Business: Seventh Edition - This is invaluable book about the music business written by a entertainment lawyer who really knows his stuff. It's an easy, entertaining read and I learned a TON about the biz from reading this one book.
Guerrilla Music Marketing Handbook: 201 Self-Promotion Ideas for Songwriters, Musicians and Bands on a Budget (Revised & Updated) - As featured in the movie 'School of Rock'! Great marketing and promotional tips.
Booking, Promoting and Marketing Your Music (Mix Pro Audio Series)