One of my favourite books on photography in general, and underwater photography in particular is The Underwater Photographer by Martin Edge.
The fifth edition of this classic is available as an ebook now, and in print (hard or softcopy) soon.
New chapters cover the latest equipment, processes, and techniques including SLR Cameras, water contact lenses, mirrorless interchangeable lens cameras, micro four third systems, super macro techniques, motion blur, LED lighting and more.
I have every edition of this book, and it has been a constant, but updating, companion in my underwater photography journey from film cameras, to dSLRs and into modern mirrorless cameras.
I highly recommend this book for those interested in UW photography, and photographers in general.
Pre-order Discount: The Underwater Photographer Edition 5
The eBook version in the above discount is available in the VitalSource eBook app. The ebook is also available on Kindle and is showing as coming in Apple Books.