Another great free OS resource is MIT's 6.828: Operating System Engineering.<p>"This course studies fundamental design and implementation ideas in the engineering of operating systems. Lectures are based on a study of UNIX and research papers. Topics include virtual memory, threads, context switches, kernels, interrupts, system calls, interprocess communication, coordination, and the interaction between software and hardware. Individual laboratory assignments involve implementation of a small operating system in C, with some x86 assembly."<p>Lecture notes from 2012: <a href="https://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-828-operating-system-engineering-fall-2012/" rel="nofollow">https://ocw.mit.edu/courses/electrical-engineering-and-compu...</a><p>Video lectures from 2014: <a href="https://www.youtube.com/watch?v=kDRHsNauoxk&list=PLfciLKR3SgqNJKKIKUliWoNBBH1VHL3AP" rel="nofollow">https://www.youtube.com/watch?v=kDRHsNauoxk&list=PLfciLKR3Sg...</a>
Note that this book is half-finished and work on it has been discontinued (as of 2 years ago). If you want a good resource on OSdev, start here: <a href="http://wiki.osdev.org/Main_Page" rel="nofollow">http://wiki.osdev.org/Main_Page</a>