I'm giving the book a provisional four and a half stars. I haven't had an opportunity yet to try the recommendations in the book on my parents' cats so I can't speak to how well they work.
Although I've loved cats all my life and subscribed to Cat Fancy when I was young I still learned some valuable information about hierarchy, territory, the importance of vertical space, and how to correctly play with your cat(s). Yes, there is a correct and incorrect way. Even if you're not having problems between your cats it's worth a read.
I will say that the book does seem geared to people with more than two cats.
The only other thing I'll mention is that I've always heard the rule is have as many litter boxes as cats +1 and Johnson-Bennett's rule is # of cats = # of litter boxes. I think the +1 rule is probably better, but it's an easier rule to follow when you only have two cats.