4/5
This book is a bit of a hard one to talk about, because a new version of the book is going to come out soon. If you are reading this, I'd recommend buying the newer edition when it comes out.
This book feels like a collection of blog posts on the topic of garbage collector design -- it first covers the 4 major algorithm types (reference counting, mark and sweep, mark compact, copying), then a discussion of it's overall merits, then a walkthrough of the more advanced sort of properties you would want in a garbage collection (like incremental, parallel, concurrent, and so on). It is a very thorough, and has some discussions of some programming-language specific features like finalizers, and finally a chapter on real time designs.
This book is also solid goldmine of citations to dive into the literature. However,because this book came out in 2012, and the citation in the book mostly range in the 1995-2009 range, which means there's around 14 years of progress you are missing out on.
Additionally, I bought the eBook version on Amazon, but that was a mistake because it's mostly a fixed-formatting export of the PDF and I couldn't read it on my kindle, and it was kind of unpleasant to read it on the desktop client. If you can't wait for the new edition, then I would recommend getting the paperback.