The Haynes manual for your car. Even if you’re not a mechanic they are so detailed they will walk you through fixing almost anything, they’re made for the laymen. I’m a diesel mechanic and even i own one for my cars.
When friends buy a new car i buy them a Haynes manual.
They don’t do them for ever single car in the world and the coverage isn’t as great on later model stuff but if you own s car 5 years or more old they’re great.