you can poke around or just ask about specific issues.
if you wanted a general book on maintenance of bally EM slots, the only one that's really worth getting is the "owners pictorial guide to the bally slot machine" by geddes/mead. It's available new for around $45, so don't pay crazy inflated prices.
documentation for the 891-1 can be found on
https://bingo.cdyn.com/slots/the 891-1 had a couple versions ... one with jones plugs and one with molex plugs. The schematic is the w-1046-xxxx file.
there's also a bally_manuals/ folder with a bunch of the parts diagram books in pdf as well as a couple general info manuals.
having said all that, what's the issue with your machine? Pictures help a lot, but can be annoying to post here due to size limitations. Video can be even better, but you'd have to put them on a file sharing site.