There are a lot of books about Breath-Work
But the best book on Breath-Work I've found is The Science of Breath by Yogi Ramacharaka aka William Walker Atkinson
For a few reasons:
First published in 1903 it is one of the oldest books on the topic of Breath-work aka Pranayama. As a result, it's copyright has long since expired and it can be easily found online as a free PDF.
The book explores the importance of breath and provides practical advice on how to use breathing techniques to improve physical and mental health.
It details breath work practices from beginner to advanced.
It covers the idea of transmutation of sexual energy which was a topic made famous by Napoleon Hill in his book "Think and Grow Rich" (1937)
The book has a mysterious, esoteric quality like it has been channeled instead of written. Yogi Ramacharaka is the pseudonym of William Walker Atkinson who was an extremely prolific author publishing over 100 books.
If you are at all interested in Breath-Work, I can't recommend this book enough.
(And if you've read it, do you agree? What other book would you recommend for beginners?)