I just read a few of it at a glance. He wrote about method of meditation to attain enlightenment (which is based on Mahasi Sayadaw vipassana method), criticized Theravadin’s 4 stages of enlightenment (Sotapanna up to Arahant) and Mahayanist’s 10 stages of Bodhisattva, but preferred Vajrayana’s 5 stage of enlightenment, and narrated his journey to Arahanthood. Among other things, he also criticized the concept of an enlightened being in Theravada (which defined an Arahant cannot do this and that, as well as he/she should be ordained in 7 days after attaining Arahanthood).
Thank you for your caution, @Gabriel_L. I didn’t know that this controversial figure is discussed in other topic linked by Bhante @Khemarato.bhikkhu above. For others, I should warn to read this book with caution and should also read the article by Bhante Analayo in the other topic which has good points from traditional Theravada view.
This book and its author has been discussed at various times on the Forum @seniya. Here is a list of search results for you to peruse. If you don’t understand the concerns that people on this Forum have you might go straight to this article: