I am not sure there is a book to show how to understand an electrical system. There are books on general principles of electricity. But they most often describe principles and not specifics. Having the service manual with the correct wiring diagram is needed to understand any electrical system.
Thinking of voltage as pressure and current as flow in a piping system is an excellent analogy for DC (direct current) electricity. Figuring out the colors in a wiring diagram, locating the correct wiring and components, then being able to measure voltages (and sometimes currents) will allow troubleshooting most any vehicle electrical system. A multimeter is needed.
The reason some makes state to run separate wiring back to the battery for accessory ground is because the frame is made of aluminum and steel. The ground current will travel through the frame and dissimilar metal locations can corrode because of the current - which then causes a failure. Also, the ground wire or grounding has the same electrical importance as the positive wire.
The best advice I can give is: figure out how to read a wiring diagram (schematic) for the system you are interested in learning. Once you can decipher the color coding, routing of wire, and components, the measurements (via a multimeter) and sorting of why something does not work becomes much easier. Also, figuring out why something works or doesn't from a wiring diagram helps prevent damage to the motorcycle. Some simple problems like a dead battery or that a switch is now in pieces on the floor can be easy to spot, but for most anything else a wiring diagram is needed.