Search for roof trusses In the library catalog and head to the shelves and browse the titles. One book, Historical American Roof Trusses, call number 721.5 .L48 2006 looks like it may be useful.
You might look at The Encyclopedia for Vernacular Architecture. It's a reference book and the call number is 721 .E53 1997.
Finally, you might check the website for the Timber Framers Guild for a glossary of timber framing terms and articles about joinery.