I read this book over Lent and learned so much from it. I highly recommend it, and think it qualifies as essential reading for Catholics. I've attached a pdf of a powerpoint I made and used for a lecture on the book. My presentation only scratches the surface of the richness of the actual text but I think it serves as a good cliff notes of each chapter. This is a book that one can return to and get something new out of every time.
This is a top notch psychology book. Modern psychology has done away with the soul and/or separated it from the mind and body. This book does not do that. He uses solid Catholic principles and Sacred Scripture to give us a practical approach to self-discipline.
He explains what Self-Knowledge actually is, the principle of self-discipline, the Four Laws of Human Nature, and much much more. I cannot recommend this book enough. You won't be disappointed.
https://archive.org/details/SelfKnowledgeSelfDiscipline1916/page/n9/mode/2up