I am using Mason for about a year. Then I saw the Mason book. My first tought was: "Why should I buy a book about Mason - The online documentation is excellent...?".
But even the introduction chapters of this book gives you new ideas how to get things done. Starting with the second half of chapter 4 every Masonsite developer should take a close look. He will find a in deep discussion about every Mason feature - and more (e.g. The Bricolage-CMS-Appendix).
I my opinion there is no discussion "to buy or not to buy" this book. The only question is "when".
On the one hand this book can be a bit boring for "new" Mason user and as mentioned before the online documentation is very good. On the other hand if you have your first mason-site done and read this book you will have very likely the urgent desire to rewrite some code.
But this is a common perl problem: "There are many ways to get things done."
I dislike the "Example" chapter. One of the big advantages of Mason is the possibility to seperate perl-code and HTML. This ist not very well done within the example-site.
Conclusion: This book is not needed to get in touch with Mason althought usefull - but if you are really starting to deploy a site I strongly recommend this book.