Gosh. Some writing, some pages, maybe art, overlooking an obvious collectible closest to "books". Where to go from here?
Photographs, possibly with descriptions or inscriptions or dedications?
I can't get past photographs containing Mary (and possibly but not necessarily the victims), that in themselves are totally innocuous, but later become significant enough to murder for. For example, they show that she was at place X at time Y when she shouldn't have been, or that she was with someone that she should not have been. Jeb Zagruder comes to mind

I'm also thinking that the five victims, Mary and Billy were at an "event", where these photos were taken, and that is all that they had in common.
Magazines, Periodicals, Comics?
Some kind of advertising or religious brochures?
Steve from Mudgee.