FWIW I continue recommend it young hackers to read/examine/study before they try to open something like the current Linux kernel.  The MIT xv6 work  to put it on an Intel Architecture just adds to the value.

As the recent thread/argument on the merits/demerits of the use/abuse of the preprocessor shows - simple and straightforward is easier to understand regardless of the arguments wrt how helpful the macros can be.   

6th Edition is clear and if you want to understand what it takes and how it works, John's commentary it difficult to be.