Some guidebooks that I found on Amazon are:

Blue Guide Umbria, Fourth Edition (Blue Guides) by Alta MacAdam (Blue Guides are excellent especially art and architecture).

Umbria, 2nd by Collective (Author), Michal Pauls (Author) (Paperback) Streetwise Umbria by Streetwise, Michael Brown (Map) Road Map of Umbria Provincial Area. Easy to Read Maps for Safe and Enjoyable Travel. (Map) Umbria: The Heritage Guide by Touring Club of Italy (Editor) (Paperback)

You might try a travel bookstore near you. I like Open Air Books in Toronto. The shop is a shambles but that's because it is crammed with travel guides, phrase books, armchair travel, etc.!

Frances Toronto, Canada