I printed the entire thread and went over all the comments one by one.
Thank you so much for your efforts! Nine printed pages. I highly appreciate this. Most of the comments are not new to me or at least very understandable. But there is a huge difference between theory and actually creating a document like this. I learned a lot, that is sure. It was really hard work, this first "book". I also had to await whether it would be failure or success, so I did not continue until all was really good. I am no pro at all, I am a car engineer. I was trained to build cars (in a big team, that is).
Some remarks:
I wanted the design be readable on all screens down to a 7" tablet. That forced me to use a font that, I fully agree, is big on normal screens. I had in mind many using the document to understand Xubuntu would like to be able to read it at the side of the screen. On a tablet or printed on paper. It is even readable on an smartphone screen, but that was not my goal.
A second consideration is to keep elder people and those with other reading issues in mind as well as the many people not very used to serious reading any more.
I considered using two columns. The width of each of the three columns exceeds that of many renowned publications and makes the text better readable.
I had several readers complain about the three column layout, although it is not that very special as a short visit to the magazine shelf of any public library will quickly show. I also wanted to avoid the impression this document was a book. I wanted it to look a bit like a magazine and reading a interesting story instead of a study book.
Finally: I appreciate any guidelines on getting really good screenshots. Mine are maybe not that nice, but they are very readable unlike quite some commercial books.
Thank you very much, also for the encouragements of which there were much more than I first thought, once I sat down to go over it all.
I welcome the ideas about something on proper layout design. It really sounds like an excellent idea!