I'd suggest "Operating Systems: Three Easy Pieces". (http://pages.cs.wisc.edu/~remzi/OSTEP/)

I've found it very accessible. Largely, the way that they teach is to describe a system based on a set of assumptions, then slowly relax each of the assumptions one by one until you reach an example of a real system. That style of teaching really works for me.

"Operating System Concepts" by Silberschatz, Galvin and Gagne, is a classic. It covers the basics: processes, syscalls, filesystems. (it's what we read in our OS course in university)

Thank you!