Unlike many other modern operating systems, UNIX was not written by a single monolithic development team. It started out as a research operating system, and its power came from the creativity of hundreds of brilliant programmers solving the problems they came across in their work. After writing a new tool, they'd typically write a reference document (a "man page"), and if the tool was significant, a technical paper describing its background and use.
For years, the technical papers (or "supplementary documents" as they have now come to be called) were the only tutorial documentation for many UNIX programs. Now, some of these papers have been superceded by in-depth books on individual programs.
However, for many programs, the Supplementary Documents remain the single, authoritative source for detailed documentation. This is particularly true in the programming area. In this volume, you'll find useful papers on such tools as the gdb and adb debuggers, source code control systems RCS and SCCS, lex and yacc, and the m4 macro processor. You'll also find such historical documents as Dennis Ritchie and Ken Thompson's classic first paper about UNIX and Kernighan and Ritchie's classic introduction to UNIX programming.
On a more up-to-date note, this book also includes a two part tutorial on interprocess communication (IPC) under 4.4BSD UNIX.