Another vote for Douglas Wile's books. He's a scholar who translates the oldest texts and puts them in context. I'd also recommend Dan Docherty's "Instant Tao" which includes some translation of poems by Li Bai. Full disclosure : trained with Dan for many years. And not Tai chi but very interesting is Dachengchuan by Wang XuanJie.
Another vote for Douglas Wile's T'ai Chi Touchstones. The intro alone is worth the price. One book everyone interested in Taijiquan should have. For applications, theory, and historical context of Chinese and Taijiquan sword use, check out Scott M. Rodell's Chinese Swordsmanship: The Yang Family Taiji Tradition. His newer e-book releases go into more detail on sword history, theory, and instruction for more advanced students. A short, useful book on theory and martial instruction is Chen Weiming's T'ai Chi ta Wen And another well-known and worthwhile inspirational/theory text is Zheng Man-qing's Cheng Tzu's Thirteen Treatises on T'ai Chi Ch'uan.
Most of the writings from 1676 to 1963 translated by Paul Brennan are available in his web-site: http://brennantranslation.wordpress.com
