Hi!
I too am looking for a decent workshop book and the only three things that seem OK are these:
https://www.amazon.de/-/en/Hans-J-Schneider/dp/3667115814/ref=sr_1_16?dchild=1&keywords=vespa+buch&qid=1616571345&s=books&sr=1-16
https://haynes.com/en-gb/vespa/et2-50/1997-2004
https://haynes.com/en-gb/vespa/s50/2007-2011
The first one, as Der Blechfahrer mentioned, should be the only one covering Primavera models. However, the differences shouldn't be that big.
I hate that I can't get the ebook version of the Schneider book, since I like the option to easily and quickly search with CTRL+F.
The other Haynes books are available as ebook or rather as a online webpage, but they are less specific and aim scooters in general.
If someone comes across any sort of a download link, maybe they could share it?