Elizabeth Zimmermann: Knitting Without Tears
This will completely change how you think about knitting. Really.
Charlene Schurch: Sensational Knitted Socks
An excellent sock book for newbies & people who want to begin designing their own.
Anna Zilboorg: Magnificent Mittens
Gorgeous. Eye candy and some good instructions too.
Jacqueline Fee: Sweater Workshop
A really good stepping stone between blindly following patterns and EZ's percentage system.
Clara Parkes: The Knitter's Book of Wool: The Ultimate Guide to Understanding, Using, and Loving this Most Fabulous Fiber
The Knitter's Book of Yarn is fabulous too!
Cookie A: Sock Innovation: Knitting Techniques & Patterns for One-of-a-Kind Socks
Really, really fun socks & great instructions for modifying your own.