Query Cache Implementation Details
This is the best documentation available for the query cache:

sql_cache.cc

I am not kidding, the header of that file contains the most detailed overview I have ever found. Next thing is the own code, which is on the very same link.

There is a query cache chapter on this book, and this other book, but only 4-5 paragraphs, in the context of its relation with the handlers interface.

Apart from that, you should read the general internals code guide.