Prism is the Microsoft Patterns and Practices Team official guidance for building “composite applications” in WPF and Silverlight.
It’s intended to provide guidance on the best practices for building large scale applications which are flexible in terms of development and maintainability.
This includes guidance on dependency injection (via Unity or MEF), layout (including using MVVM), composite event handling, etc.
Is Prism literally just (or mainly) a booklet, as given on the MSDN site? Does it include any libraries, if so for what exactly?
It is a book, but also includes the libraries for support (on the GitHub site).
Is Prism an MVVM framework? Should I be using it as the “official Microsoft MVVM framework” over others out there or my own?
Yes and no. It includes guidance for using MVVM, but is far more than an MVVM framework. It’s really not, in and of itself, an “MVVM framework” – though it does include MVVM guidance.
Does it provide dependency injection? Does it relate to MEF at all in this way?
Yes. It originally included Unity, but the latest release includes using MEF for DI.
Anything else I should know about what Prism does, as a WPF/Silverlight developer.
It’s worth taking a look at how they do things. They have a very elaborate “region” concept, as well as good event aggregation support. It also includes good practices for developing against WPF, Silverlight, and Windows Phone simulataneously.