Could it be that this represents a system for generating music?

[Mauro]

Sorry if it was explained somewhere but I can't find it: what is a "ur-book"?

He means an original book (in some language) which was 'converted' to Voynichese. I think 

Unsolicited comment: from my point of view the existence of one or more ur-books is a possibility. It has the advantage, given every languages is quite varied, to have a lot of diversity to start with, which accords with the VMS being built 'mostly' with a few core rules but then with lots of exceptions which are so hard to make a sense of. This fact may be explained by Tortsen's copy&modify or quimqu's Skribe (which partially reuses the same mechanism as Torsten's),  due to the human factor, but I think it might also be explained by an ur-book plus a sufficiently weird encryption with parameters varying between sections (the jump is too great). Possibly even varying gradually page by page inside each section. Looks less probable to me at the moment, but not by much. An ur-book might work also with a phonetic or anyway a weird notation of some language, but I see this less probable than both the previous alternatives because the differences between sections imply different ur-languages (as different as Italian is from Latin), or multiple radical orthographic reforms (at a point, it gets borderline with the cipher).

(There are of course more alternatives, ie. a weird astronomical notation, but I can't really rank their probabilities)