I can highly recommend this book to add to your internal studies. The content is for advanced practitioners and may be hard to understand for beginners, I found myself returning to this book multiple times to understand every time a little bit more.

Nei Gong: The Authentic Classic, (Nei Gong Zhen Chuan) is an obscure text of unknown origin, yet it stands alone as the definitive text on internal energy and the generation of internal power. Hand copied, and passed from teacher to student; it is one of the “secret transmissions” of Chinese martial arts. Nei Gong: The Authentic Classic, is a practical manual on internal energy development and its fundamental importance in the martial arts and fa jin (emitting force). It draws heavily on Daoist meditation and alchemical practices which are said to promote health and longevity. The text is divided into four sections: 1. Nei Gong Jing (Internal Energy Classic); 2. Na Gua Jing (Received Trigram Classic); 3. Shen Yun Jing (Transported Spirit Classic); 4. Di Long Jing (Earth Dragon Classic), which is meant to be read in order. In this way, the reader is guided step by step through the details of Nei Gong practice, and in the Di Long Jing, its application to self-defense. Translated by a team of experts, the text is accompanied by extensive footnotes and diagrams that clarify the more obscure passages which otherwise pre-suppose a knowledge of Daoist texts and imagery from the I-Ching (Book of Changes). The appendices further explicate the relationship of Nei Gong: The Authentic Classic to the internal martial art Xing Yi Quan.