No author had more impact on our understanding of the internal martial arts then did Sun Lu Tang (1861 - 1932). This Grand Master of all three arts broke with tradition and wrote down—in classical Chinese characters (characters that he taught himself)—the practice methods of all three arts.
I have never read a book about the "neija" that did not quote his work. Sun Lu Tang was a renowned master of Chinese martial arts and the creator of Sun Style Taijiquan. He was an accomplished Confucian and Daoist scholar, and contributed to the development of the internal martial arts through his published works.
Sun Lu Tang's first book--A Study of Form Mind Boxing (1915)--illustrated the complete training method for Xingyiquan. In this book he made the argument that literary and martial art learning was the same, and that training in martial arts benefits the health of the practitioner.
His second book--A Study of Eight Trigrams Boxing (1916)--was the first book on Baguazhang. In this book, he connected the physical forms to the diagrams and philosophy of the Yi Jing.
His third book--A Study of Grand Ultimate Boxing (1921)--detailed his own Taijiquan form, and forever categorized the martial arts of Baguazhang, Xingyiquan, and Taijiquan as internal martial arts.
It is difficult to determine if the division of Chinese martial arts between external and internal forms existed before Sun Lu Tang. In his book on Taijiquan he traces the development of martial arts to “ancient times,” saying that martial arts were developed to prevent the body from “growing weaker by the day, and the hundred illnesses invading.” He further says that exercises created by Bodhidarma at the Shaolin temple were the basis for Yue Fei’s and Zhang Sanfeng’s martial and qigong systems. He joined Yang Chengfu and Wu Chienchuan--of Taijiquan fame--on the faculty of the Beijing Physical Education Research Institute where they taught the internal martial arts to the public. He published two more books before his death in 1932.