Get one of the textbooks that covers MINIX, a Unix-like kernel and operating system that was the initial inspiration for the Linux kernel itself. I don't have any recommendations. but check out the MINIX website, there ought to be some information there. MINIX was initially created as an implementation of the practices recommended by Andrew Tanenbaum (its creator) detailed in his textbook Operating Systems: Design and Implementation, so it should be a particularly good codebase to study.
Edit: Check out the MINIX 3 documentation page. It looks like Tanenbaum's book is relatively up to date (last edition published in 2006) and covers MINIX 3, so this should be a good starting point.