If the 15 Minute Tai Chi Book is one that you can pass around then 'Revelations, Tai Chi Chuan, Principles and Concepts' is one that will support a lifetime of practice, in my opinion. It is a collection of writings from Grandmaster Ip Tai Tak. It is not a book that you read from cover to cover, put it down, digest it for a time and rarely return to.
Profound books on Tai Chi are often impenetrable, but this one, I feel, is accessible enough. This book mainly shares Master Ip's insights into the practice and the written classics on Tai Chi. There are not many long passages that breakdown Tai Chi concepts in a wordy fashion. Master Ip has done most of that work for us, so this is a distilled study aid created to hold the hand of our mind while our body digs deeper into the practice. Master Ip's content is more thought and practice provoking than explanatory.
Most of the content of the book is taken from his memoirs. His expansion on Yang Cheng Fu's ten principles is clear, concise and practical. The movements of four forms are listed including the long form. There is an interview at the end of the book conducted by his first disciple, MDA founder, Master John Ding. If there is one thing missing for me, it's that we do not get to deeply into Master Ip himself, which in turn is, I suppose appropriate as he was all about the practice. The sixteen chapters in this book contain some, not all, of Master Ip's insights from a lifetime of devout hard training expressed poetically without being fluffy, practical without being dour. It is one I highly recommend and return to again and again and again. Available here