I learned about Qi and Jin generation from various TJQ masters, private lessons, seminars, from manuscripts, books, videotapes, and so on. All (useful) that I learned I incorporated in my TJQ practice.
Different sources explain the same things in different ways. Also some methods are very simple and effective while others are very complicated and ineffective. No source that I experienced explained the »internal« part of TJQ enough completely, but I can make a complete knowledge about the internal part of TJQ only with combination of more sources.
In my searching of a complete source (book) about the internal part of TJQ I didn't found yet souch book. Many books have very interesting explanations, but also they are incomplete.
I began to write my own manuscript on the internal part of TJQ
I will make my comments (and explanations) on the more important Taijiquan principles and Classics quotes. No book more needed
P.S.: Today I received the book Taijiquan Wuwei: A Natural Process by Kee-Jin Wee (<-Huang Sheng-Shyan <- Cheng Man Ching <- Yang Cheng Fu] and it is one of the best (if not the best) book about the internal part of TJQ. I highly recommend it (temporaly the english version is out of print)!