I haven't been aware of the fact that quite a few books by Niklaus Wirth are available as free downloads by now. Among them are such classics as Programming in Modula-2, Algorithms and Data Structures, Compiler Construction, and Project Oberon, all of them either originally in/about Oberon, or adapted to Oberon.
These books have influenced me a lot. Niklaus Wirth's main strength is a focus on the essentials, which is especially apparent in his concise programming style. I can especially recommend Compiler Construction which basically contains all you need to know about compilers in a very thin book (unless you decide to make this your field of expertise), and Project Oberon which contains the complete source code of a full operating system (including a file system, a garbage collector, a GUI, and so on).
See also the ETH Oberon Home Page for more information about Oberon and related materials.