The book I carry with me to all workshops and refer to the most is:
[h=3]
All-In-One Quilter's Reference Tool Easy-To-Follow Charts, Tables and Illustrations, Yardage Requirements, Cutting Instructions, Setting Secrets, Choosing ... Piecing Techniques, Number Conversions by Harriet Hargrave, Sharyn Craig, Alex Anderson and Liz Aneloski (Spiral-bound - Oct 1, 2004) [/h]It has all the basics in an easy to read, small and easy to carry spiral bound book.
My second favorite reference book is:
Patchwork Minus the Mathwork: A Quilter's Guide to Planning and Buying Fabrics without a Degree in Mathmatics! by Jeanne Stauffer (Paperback - Nov 20, 2009)
This book is an excellent reference book for how much fabric is required for the different parts and sizes of a quilt. It also has the quantity of pieces of a certain shape that you can get out of a specific amount of fabric. I refer to it quite often as well.