The original books were written in the Oberon System Word Processor, and you can access them as part of Oberon live documentation.
For example by running A2 and starting Oberon System 3 from it.
Regarding converting them into epub, I guess it is a question of copyright, even if Wirth has made the PDF versions freely available, but I am no lawyer.
If you or anyone reading this has access to that book in epub or similar ereader format I'd greatly appreciate. It seems Wirth published only a PDF (probably from MS Word)