If you've just found out about Boost.Build V2 and want to know if it will work for you, start with Chapter 20, Tutorial. You can continue with the Chapter 21, User documentation. When you're ready to try Boost.Build in practice, go to Chapter 19, Installation.
If you are about to use Boost.Build on your project, or already using it and have a problem, look at Chapter 21, User documentation.
If you have questions, please post them to our mailing list, and be sure to indicate in the subject line that you're asking about Boost.Build V2.