There is an article up on the BBC with an interview with John Haynes, he of the ubiquitous repair manuals.
It was not long after I got my first scooter that I purchased the Haynes manual for the Vespa. There are two for the classic Vespas; one for the P-series, and one for all the other non-P rotary valve Vespas.
Neither of those manuals are perfect. However, they are a good start... and back in the days when there was no internet and virtually no technical information on Vespas available in the US, they were all there was. They are both useful to have on your shop bookshelf, no doubt. Thank you Mr. Haynes.