Start skimming through the included documentation files. You don't have to do a thorough read, but the Documentation folder is structured in a way mirroring the organization of the kernel. They will help you to understand the structure, and you can get deeper understanding of the essential parts that way.

Once you have mental map of major areas of the kernel it will be easier to relate to the sourcetree.

There used to be a number of great books on fairly esoteric issues of the kernel (like, comprehensive explanations of network stack). However most of them seem to be stuck at 2.6 and are no longer very up to date.