Lorna Mitchell has posted a book review of an O'Reilly publication (by Steve Francia) - "MongoDB and PHP".
The tone of the book is quite informal and approachable [...]. This book also does a great job of deferring to other resources where that is appropriate; in particular where some features of MongoDB will change rapidly and the online documentation is well-maintained. [...] There is no hiding from the very lovely, very technical, features available in MongoDB and although this book doesn't drill into all the possible use cases of each one, it does cover some advanced topics such as sharding and GridFS which was completely new to me.
Her review of the book is positive, noting that it takes a topic that, despite possibly being overwhelming for someone new to it, makes it approachable and easy to digest. You can find out more about this book on the O'Reilly website.