By Andrew S Tanenbaum, Albert S Woodhull; Prentice Hall, 2006, ISBN 0131429388. Revised simplified edition, for latest MINIX version. An OS text which first explains relevant principles, then demonstrate their use via Unix-like OS as a detailed example.
Line-by-line comments of source code for TCP/IP networking protocols, network service, for Minix version 2.0.4. By Christos Karayiannis, Andrew Swartzbaugh.
Unix clone that is free, small, flexible, secure, and self-healing, leading to high reliability, even during software faults. Descriptions, FAQ, book, newsgroup, screenshots, bug reports, downloads. [open source, BSD-like]
MINIX that runs atop Mac OS. Benefits: while learning about operating systems, its easy to recompile and relaunch, with no rebooting; and to keep OS backups in separate files.