Lions' Commentary on UNIX 6th Edition, with source code
 

It becomes hard to believe that some of the most advanced, modern computer systems in the world are spawn of a single operating system developed in the '60s and 70s. UNIX was an AT&T creation, and was the base for some of the most influential software today.

Lions' Book is a commentary on the 6th Edition of UNIX. Through the book he takes you through some of the more significant points of UNIX, including the main() procedure, startup, processes & management, hardware traps and interrupts, system calls, I/O, file control, file systems, pipes, and interactive terminals. The book's source code which is essentially UNIX VI at the core is commented by Lion himself, and as the book goes on he talks about procedures in the system and how they work such as the memory functions (malloc and mfree are the first two functions described.)

The first half of the book is roughly an Introduction to UNIX, an intro to the PDP-11, and reading C programs. Sections One through Four take you through UNIX VI itself, by describing what happens. Lion gives full yet readable explanations of the functions covered (despite the fact that Unix VI contains the legendary comment "You are not expected to understand this.") While Unix 6 was originally released little over 30 years ago, Lions' book stands as an excellent reference to one of the biggest operating systems of its time and a good documentary on some very interesting code in general.