Vogue Knitting: The Ultimate Knitting Book is the book I turn to for the basics– casting on, binding off, seaming, increases and decreases.
The Spinners Book of Yarn Design by Sarah B. Anderson. In a crowd of vague and opinionated books on spinning, this one stands out for its straightforward instructions and nonjudgmental attitude. If I only had one book on spinning, this might well be it.
Color in Spinning by Deb Menz; an amazingly well-researched and written book that helps spinners understand color at the finest level. A great resource for dyeing your own fiber as well.