The National Preservation Trust, a non-profit organization in the U.S., has a couple of pretty cool
free books about barns available on their website. With the organization being dedicated to the preservation and restoration of old buildings, these books are primarily about . . . you guessed it . . . the preservation and restoration of old barns.
As you might suspect, both of these ebooks are in the pdf format.
There are lots of cool photos of old barns in these books, but you won't mistake these books for being coffee table books. And, these books are much smaller than most coffee table books. But, then, you don't have to pay for these like you would most coffee table books, either.
Barn Again! A Guide to Rehabilitation of Older Farm Buildings.
Historic Barns: Working Assets for Sustainable Farms.