I think this stuff is reasonably simple (if intricate) but I've been doing it for years and it wasn't easy at the start. Also I've had bitter arguments here with people who claim to teach chemistry which arguments revealed that they were not capable of understanding it so maybe I'm some sort of natural chemical genius but a quick discussion with my wife would certainly change your mind if that's what you were thinking. WRT the water book: John didn't want to put too much of the 'meat' into so if you find it difficult that implies that you have a rough road ahead.
I guarantee that that is indeed the case. As Levine says in the introduction do his p-chem book, you don't get into shape by reading a body building book - you have to do the exercises.
You need to know some basic (highschool/college freshman) chemistry i.e. what electron, protons, atoms and ions are, how to read a chemical equations, how the strengths of solutions are specified, a little about acids and bases and, the tough part, something about chemical equilibrium equations - the part of your chemistry courses you hated most if you took any. You'll also need some math i.e. the ability to work with logarithms and anti logarithms. IOW, engineers and scientists are going to have an easier time of it than poets and philosophers. I'd suggest looking at the Wikipedia article on Henderson - Hasselbalch Equation. It is the basis for all this stuff. If you can read and understand that then you should be able to follow what's at
https://www.homebrewtalk.com/forum/threads/calculating-bicarbonate-and-carbonate.473408/ which is the only difficult part of brewing water chemistry in a nutshell. If there are parts of the Wikipedia article that puzzle you they would suggest where you need to go to build the required fundamental knowledge base.
With respect to a book on how to do the calculations: I've thought about it and would write it if I thought anyone would read it. Very few people are interested in such things. I put up the linked post on how to do the calculations about 3 years ago. In it I told how to do them but not why they were done in this way. I offered to explain the why's and wherefore's to anyone interested but have had enquiries from very few people.