Reddit mentions of Automotive Electrical Haynes TECHBOOK (Haynes Repair Manuals)

The trickery in those systems is not in their electrical wiring, but in that they're increasingly computerized. ABS and EFI in particular. With increasing model-specific complexity also comes increasing variety, so you're going to need a model-specific service manual.

If you're unfamiliar with 12V electrical systems in general, I'd probably start by reading the generic automotive electrical books. Haynes would probably suffice to get you up to speed, so that terminology used in the service manual makes more sense.

The main differences you'll find will be in charging systems and accessory wiring (both covered in a model-specific service manual), but pretty much everything else follows a similar (+)->Distribution/Fuse->Device->Ground circuit.

Where it gets complex is the ECU. I know a few bikes use a variation of OBD-II (Triumph comes to mind), but the rest are completely proprietary, requiring their own set of scantools and protocols. Pretty much all EFI and ABS diagnostic procedures will be done with one of these scantools (or at least pricey cable and software) if you want live or freeze frame data where supported.

The same trend continues in the automotive world as well. ABS and electronically-controlled transmissions were a big deal when they first hit the market (pre OBD-II), and now they're adding drive-by-wire and hybrid technologies to the mix. The only thing standard between manufacturers and even model years is engine management data (emissions). Airbags, ABS, SRS, and body electrical control data still require dealer scantools to access. They don't make their PIDs public knowledge, so these scantools are often priced out of reach of all but the big shops.

Finally, not that you're gonna botch anything, but electrical is only worth doing if it's done right, so take pride in your work! No half-ass duct tape and twist splice crap. I've had tons of vehicles come to me in sorry shape after being "fixed" with electrical tape and prayers, many of which have required a new harness ($$$, time, patience, sanity). Doing it right takes skill and patience, but it's worth the reward when your car/house/bike doesn't burn down in the course of normal operation :)

Sorry, that was long.

TL;DR: It's not much different from cars; computers are weird

Here's a good primer.

When you start getting much more complex than that, it starts getting manufacturer specific... So unless you have a specific manufacturer picked out, I'd just learn all you can about electricity in general. You'd be surprised how many problems can be fixed with five minutes and a soldering torch that would take most mechanics a hundred dollars in new parts.