10-15 years ago, if you bought an old Vespa, the next thing you bought was the Haynes Manual. You had two choices: one covered P-series Vespas, and the other covered all other rotary-valve Vespas. They were limited in scope, contained a few errors, and the language was often confusing to American scooterists (“look inside the gas tank with a torch,” or “secure the new item via the Douglas spares replacement scheme”) but they usually got the job done. Since then dozens of scooter books have been published, but the Haynes books are still indespensible to vintage Vespa owners.
Since then, Haynes has added more scooter books, notably one for Vespa and Piaggio automatics and “The Scooter Book”, a general reference for scooterists. Their latest book, Twist and Go Scooters Service and Repair Manual contains specific specs for a wide range of scooters, as well as mainenance and repair info that applies to all automatic scooters in the 50-250cc range. Haynes’ US distributor does not appear to stock any of the more recent books, but some are available on Amazon, and the rest are available directly from Haynes.