My best advice would be to master (if that is at all possible) Photoshop before turning your attention to this book. There is no doubt that the authors understand the subject and have also found the right pitch in order to share their combined expertise with the reader.
I have a number of good quality colour slides which were never used because of something - such as a diver's fin, in the picture. The first lesson I learned from this book was how to remove those unwanted items in such a way as to leave no trace of any alteration.
My subject, however, is shipwrecks and high quality photographs of, for example, the entire stern or other large section of wreck, often elude me. Consequently, the content of page 99 jumped out at me. This shows a fairly good quality photo of the stern of a large shipwreck surrounded by 5 divers with even more in the far distance. With carefully detailed instructions, this picture is changed from something which was good into something quite spectacular.
Having read that particular page several times, I then played around with some of my own photographs and, to begin with, ended up overdoing it. Of course the advantage of computer enhanced images means that none of the original detail was lost and I was able to start over. My point being, I would caution the reader not to overdo their alterations to photographs.
Altogether, a very good product and one which has a definite place on the bookshelf of all serious underwater photographers.