You can download a number of books on the Oberon system, designed at the Compter Systems Institute at the Swiss Federal Institute of Technology. This project was launched in 1985 by Niklaus Wirth and Jurg Gutknect to build a single user, multi-tasking operating system from scratch. The resulting software is available under a straightforward BSD-like license, and you can download many of the books on Oberon and its compiler as PDF files. Lots of good reading from an alternate path in the world of compilation and operating system design. Via Lambda the Ultimate
Oberon was an important influence on Plan 9, and particularly on the ACME user interface. It had the first bitmap+keyboard+mouse IDE, and still a whole lot better in many ways than most of what you’ll see these days.