Equally or more valuable than trying and sit down to read the kernel source is to follow the patches and discussions thereof on the linux-kernel mailing list: http://lkml.org/
I've found that doing so a) improves my knowledge of what's going on under the hood tremendously, and b) has taught me a fair bit about how to do code reviews and make difficult design decisions. Studying the kernel itself line-by-line would take ages, but reading a few threads a week depending on what piques my interest is totally manageable.
I don't treat is a novel, but a reference. I'll often wonder how certain things are implemented, and I'll poke around for the data structures and algorithms until I gain some understanding of how it works.
I've found that doing so a) improves my knowledge of what's going on under the hood tremendously, and b) has taught me a fair bit about how to do code reviews and make difficult design decisions. Studying the kernel itself line-by-line would take ages, but reading a few threads a week depending on what piques my interest is totally manageable.