I doubt there's a book just on relations. There's justnot that much there. But there is plenty info in the ProHelp that comes with pro/e. In the Index, type in relations. There are dozens of topics. Functions used in relations will be your most valuable page, along with strings as arguments. You basically have If, Then, Else, and functions you can call. No loopingor function creating. There are relations in models, repeat region relations, drawing state relations, etc. I'm referring to Proe2001 info here.
what release was Pro/E at back in '96? I'm having trouble
figuring out how to edit Pro/Program with 2001 more than one line at a
time. A computer text book that's nearly a decade old isn't going
to be of much use. Must not have had a big demand if it
never got past Edition 1.