The "Linux Insides" book is a great effort, however I consider it to be of relatively limited use for conceptual understanding. It's more an annotated tour of the source code, read out in a literal fashion. Probably quite useful for aspiring kernel hackers to get acquainted with key data structures and subsystems, but not much in the way of
a priori knowledge.
The LWN kernel index (https://lwn.net/Kernel/Index/) has always been much better at bridging the source code and the theory into good technical articles.