Silently store up knowledge and ponder. One can gradually arrive at the heart's desire......The practitioner must carefully study. This is the treatise.
                                                                                                       (Wang Tsung-yueh)

Tai Chi for Everybody
by Karel and Eva Koskuba, published by Readers Digest, is most  highly recommended.

This book has the complete first section of Lao Jia Yi Lu as taught by the Three Treasures School so is an invaluable tool for students. It also has detailed instruction on many other aspects of Tai Chi including silk reeling.

Introductory books for beginners:

Movements Of Magic  Bob Klein, Frog Ltd
There Are No Secrets Woolfe Lowenthal, North Atlantic
Gateway To The Miraculous Woolfe Lowenthal, North Atlantic
Tai Chi Chuan For Health And Self Defence T.T. Laing, Vintage Books
Chen Style Taijiquan (The Source of Taiji Boxing)  Sim and Gaffney, North atlantic books

Intermediate and Advanced Reading:

The following books are important reading for those who develop a strong interest as often these are translations of original works (Classics).

Chen Taijiquan: Masters & Methods  Davidine Sim & David Gaffney, D&D Publications 
Cultivating The Chi Stuart Alve Olson, Dragon Door
The Intrinsic Energies Of Tai Chi Chuan  Stuart Alve Olson, Dragon Door
The Essence Of Tai Chi Chuan The Literary Tradition (Taiji Classics) Lo/Inn/Amacker/Foe, North Atlantic
Tai Chi Touchstones   Douglas Wile, Sweet Chi Press
The Tai Chi Boxing Chronicles Kuo/Guttman, North Atlantic Books
The Five Levels of Taijiquan    Grandmaster Chen Xiaowang, Singing Dragon Books

Other recommended books include:

Mind Over Matter; Higher Martial Arts Shi Ming/Siao Weija, Frog Ltd.
Chen: Living Taijiquan in the Classical Style  Master Jan Silberstorff
Chen Familly Taijiquan Form Manual   Contact three treasures taiji for details