A better question is 'did this book have to be written?' and with most of what you see in the bookshop window the answer is 'no'. If the answer is 'yes' then I

A better question is 'did this book have to be written?' and with most of what you see in the bookshop window the answer is 'no'. If the answer is 'yes' then I can tolerate the single idea taking up a whole book. If the author then goes on to write a second, third, fourth book then they have probably crossed over from writing books that needed to be written to just regurgitating the same idea, based on the fame of the first book. Publishing is an industry and they know how to do marketing.