A bit underwhelming after coming from the comprehensive Open Water Diver Manual. I guess there really is not much theory to recreational diving after the Open Water Diver certification, aside from the deep diving section of this book. It reinforces the diving physics that was discussed in the entry-level course and is a good reminder of what happens when you go deeper than 20m. The "elective" adventure dive sections of this book are like primers for the other "specialty" courses that PADI sells, there are graded exams at all and are optional.
Excellent book for advancing in scuba certification. I read over half of it and achieved my Advanced Open Water Diver Certification from PADI (Professional Association of Diving Instructors). I'll continue to read more as I work toward higher cert levels. Rather dry reading. Feels very spoon-fed. But this is safety stuff that you don't want to joke or be opinionated about. Well-integrated learning material that you can do from home.