When we bought our
boat, we found on board an excellent instructional book, 12v Bible I think, which was well written, had no stupid
mistakes (like stranded wire carries more
current due to skin effect, thank you Painless hotrod
wiring 
) and was easy to understand. That and a Harbor Freight 6 buck multimeter will help with a lot of troubleshooting.
I've been in
electronics for over 60 years, so things that are obvious to me are black magic to most people, so my wife smacks me a lot when I start talking down to her or others.