Unix

“This is the Unix philosophy: Write programs that do one thing and do it well. Write programs to work together. Write programs to handle text streams, because that is a universal interface.” — Douglas McIlroy

Unix is not just an operating system, it’s also a (software design) philosophy:

Make each program do one thing well. To do a new job, build afresh rather than complicate old programs by adding new “features”.

This is often summarized as do one thing and do it well and it’s used as a synecdoche for the Unix philosophy as a whole.

Expect the output of every program to become the input to another, as yet unknown, program. Don’t clutter output with extraneous information. Avoid stringently columnar or binary input formats. Don’t insist on interactive input.
Design and build software, even operating systems, to be tried early, ideally within weeks. Don’t hesitate to throw away the clumsy parts and rebuild them.
Use tools in preference to unskilled help to lighten a programming task, even if you have to detour to build the tools and expect to throw some of them out after you’ve finished using them.

Amen!

History

The Evolution of the Unix Time-sharing System (DM Ritchie, 1984) describes the humble beginnings on the PDP-7 computer (even before the introduction of /path/names), why fork and exec ended up being two distinct syscalls and how the pipe was invented. It covers the period from 1968 to 1973.
Ken Thompson interviewed by Brian Kernighan is a great interview with one of the authors of Unix.
The Unix Time-Sharing System (DM Ritchie, 1974) describes Unix as it was cca. 1974. This is a very widely known paper.
Unix Implementation (K Thompson, 1978) describes the implementation details and rationale behind them.
In praise of Plan 9 by Drew DeVault is a nice introductory blogpost about Plan 9, the successor of Unix developed at Bell Labs by an all-star team including Ken Thompson, Dennis Ritchie, Rob Pike and others.