Welcome to my world.
I am sure you have seen this site:
FOP: Embedding
XSL-FO Input
FOP Examples
And I found one book:
"Professional XML Development with Apache Tools: Xerces, Xalan, FOP, Cocoon, Axis, Xindice
by Theodore W. Leung " It has one chapter devoted to FOP.
What I am doing is using the demos in the downloads and an appView that came with it and just trial and error. It is painful but it is pretty much what I need for a printout I have to do.