The practitioner must carefully study. (Wang Tsung-yueh)

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

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

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

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 for intermediate/advanced students
The Inner Structure of Tai Chi Mantak Chia/Juan Li, Healing Tao Books
Advanced Yang Style Tai Chi Chuan Vol 1 Dr. Yang Jwing Ming, YMAA Pub. Cnt.
Mind Over Matter; Higher Martial Arts Shi Ming/Siao Weija, Frog Ltd.
Chen Style Taijiquan – The Source of Taiji Boxing Davidine Siaw-Voon Sim and david Gaffney, North Atlantic Books
Chen: Living Taijiquan in the Classical Style  Master Jan Silberstorff
Chen Familly Taijiquan Form Manual   Contact three treasures taiji for details

There are other interesting books on the market. Avoid those that are glossy and those that are mostly pictures of the form (they will invariably be different to the form you are studying, and it is very difficult to learn from them)