This is a well-edited and properly scholarly contextualized collection of Yang family primary source materials on Taijiquan. The majority of the material is transmitted directly from Yang Cheng-fu if not featuring him specifically (photos, demos, discourses, etc.). Though there are some lineages of non-public variations of Yang family TJQ, this book does not delve into those, but the conceptual discussions and transmitted sayings still apply across variations and contrast functionally with the sorts of fundamental concepts found in other systems like Xingyi, Xin yi, Bagua, Chen TJQ, etc.
There are a handful of other books that exist which are just snippets from this one. If you have even a marginal dedication to Taijiquan or any other internal Chinese martial art, you probably already know this one. It's among the handful of mandatory books and honestly remains amazingly accessible. Ideally somewhere high-quality versions of the images with Yang Cheng-fu exist and will be widely republished or made available.