The principles of Tàijíquán are described in the following six Tàijí Classic Texts, upon which all Tàijí systems are based:

The Chang San Feng Tàijí Classic

The Wang Ts’ung Yueh Tàijí Classic
The Song of the Thirteen Postures
The Understanding of the Thirteen Postures
The Song of Tuīshoǔ
The Song of Substance and Function

For the Yang style and its derivatives there is then:

Important Points of the Yang Family

And for systems such as ours, from Huang Sheng Shyan, there is also:

Huang Sheng Shyan's 20 Important Points

To properly understand Tàijíquán it is essential to study the Tàijí Classics, and apply their interpretation into your daily practice of exercises, Forms and partner trainning.

For a translation and detailed discussion of the meaning of the Classics, please read Wee Kee Jin's book "Tàijíquán - True to the Art".  It can be purchased online from Jin at his website.