The Art and Craft of Papermaking by Sophie Dawson is my favorite papermaking book.  It takes you from great step by step instructions on the very basics to explanations of works by all kinds of creative paper artists.  ISBN 1561381586

Papermaking with Plants by Helen Heibert is great if you want to work with natural fibers.  Again it has great photography as well as easy to read instructions.  Rumor has it that she is also working on a pigment book but it is not out quite yet. ISBN 1580170870 - She also has a new book out called the Papermakers Companion.  It has a lot of information but pictures are small and hard to follow.  

Papermaking - The History and Technique of an Ancient Craft by Dard Hunter is THE book if you are looking for information about the history of papermaking.  ISBN 0486236196

The Cloudmakers by James Rumford is a darling children's book.   It is a cute story about how a boy and his grandfather take their shoes off and make paper out of their sandals to save their lives.  ISBN 0395765056

Paper Art - The Complete Guide to Papercraft Techniques by Diane Maurer-Mathison ISBN 0823038408

Cover to Cover - Creative Techniques for Making Beautiful Books, Journals and Albums by Shereen LaPlantz ISBN 0937274879