Oxygen Advantage is a great book. I'd recommend James Nestor's - 'Breath' [0] as a first read, however. I feel like the book has more real world application and I picked up a number of things outside of what I learned studying Hof breathwork. I also studied, prior to both books, with Wim Hof in Poland in 2017 on one of his week long expeditions. The in person experience of group breath work is something to experience if you've never. It's so much different than working on it solo - especially if you're new to any of these methods.
Breath work and cold exposure have been a large part of my life after reading 'What Doesn't Kill Us' by Scott Carney [1]. That was the book that drove me want to experience it first hand.
[0] https://www.goodreads.com/en/book/show/48890486-breath [1] https://www.goodreads.com/book/show/30039048-what-doesn-t-ki...