This is a very easy-to-read book on the implementation of xv6, which is a basic unix-like operating system written for educational purposes. xv6 itself is a very simple and straightforward kernel and the source code can be found here.

I've been reading it casually over the past few weeks and found that it helped me get a better understanding of many basic operating system concepts. I've also enjoyed reading the source code to understand what a basic implementation of common system calls could look like.