The Moto Guzzi Story by Ian Falloon ISBN 1 85960 414 5 first published in 1999 by Haynes (as in the manuals) Publishing. My copy bought for me in 2005 is the second edition (June 2000) with additions was £19.99 and worth every penny!
I also have Moto Guzzi Sport and Le Mans Bible by the same author ISBN 978-184584064-8 published by Veloce Books another good book but does not cover the small blocks.
Best wishes