The defacto guide to Unix
based Operating Systems
. Written by Andrew S. Tanenbaum
, it was first published in 1987
OS:DI was the book in which Tanebaum designed, wrote-out and explained Minix, the predecessor to Linux and many other OSs.
Currently in its second edition (ISBN 0-13-638677-6), OS:DI is published with the full source code (as the original) of Minix along with a full explanation of all the principals (theoretical as well as practical) of a true multitasking OS.
An absolute must read for anyone wishing to understand the underlying structure of operating systems. It should be a required study for any Computer Science Major.