The best is any modern wiring diagram and study it until it makes sense. Toyota's are pretty good.
The answer is not a book. Maybe in the old days, but not anymore. To understand wiring you have to first understand what travels wiring- electricity. You can't fully understand something that moves by studying a picture that stands still. YouTube. That is your answer. A video demonstrating the flow of electrons and electrical theory will benefit you greatly. Might I suggest my old favorite? Howstuffworks.com. Go forth neophyte!
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