Not a book, but you could try Hakan Gustavsson's website:
http://surfcity.kund.dalnet.se/scw-1937.htm
He's also an author and a frequent visitor to this forum.....could try and convince him to write the book?
Air War over Spain (Jesus Salas Larrazabal)
Combat over Spain (Duke of Lerma)
Both books are in English. The first covers the entire conflict in the air with a slight leaning towards the Nationalist Forces in perspective. The second is more a biography of a Nationalist pilot. Neither are specifically about the Italian volunteers but do cover them in some detail.
If you read Italian, then there is:
Italiani Nell'aviazione Repubblicana Spagnola by Angelo Emiliani
I've only seen extracts and there are also extensive photos of Potez 540 bombers (Republican forces?) and the translated bio refers to Italian volunteers, which I assume from the title - for the Republican side.
Good luck in your search,
...geoff