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.