It's not a book, but I can highly recommend a youtube channel called ScannerDanner. The lessons are GREAT! He has a paid video course which I hear is fantastic, but the free videos are a great start for most shade tree mechanics. The course is more for actual technicians who make their living based on what they learn. Combine those lessons with a 2-year subscription to alldata.com for your car. The corporate accounts are like $200 a month, but you can get a 2-year subscription for a single vehicle model for just $30. You might also try identifix.com. It includes interactive color wiring diagrams; specifications for all the torque measurements, fluids, etcetera; part numbers; component locations; repair and replace procedures; and real world repair solutions to various problems. There is other stuff too, but that's what I can think of off the top of my head. You can print to pdf files as well. Sadly, you lose the interactive aspect of the color wiring diagrams and repair sheets when you download, but they are still the best wiring diagrams you will ever find.

A $5 test light & these wiring diagrams will help you solve 90% of all electrical problems. Add in a decent DMM and some jumper wires, and you've got the rest. There are other cool things that might save you some time/money in certain situations like a DC ammeter and inline resetting circuit breaker for finding shorts, a NOID light for injector pulse, a scan tool for DTC and other code checking...if you're really spiffy you can get one of those scan tools like a Snap-On Verus which lets you do all sorts of cool stuff like activating relays or components remotely with your little tablet. There are even probes that give you access to oscilloscope functions for compression, injector pulse, spark, sensor signals, etcetera. The only problem with the Verus and similar tools is that the cheapest you'll find one for is about $800. But it is REALLY cool! Seriously though, a test light will do like 90% of what you need.

My suggestion is a test light (maybe one with a switch to go back and forth from hot to ground quickly for added convenience, but those are like $40 - $50 instead of $5), a decent DMM that can at least measure DC 10A, diodes, and Hz like a Klein MM600 (you really don't need RMS unless you're doing AC circuits), and an old school analog induction ammeter for helping you locate shorts (You hold it next to the wire or wire bundle, and the needle deflects in the direction you need to look. Left or right. Upstream or downstream. It works because shorts draw more current, so the needle will deflect in that direction).

With the test light, DMM, and ammeter you can easily check all the common electrical problems in a car. Open circuits (a break in a wire), Short to ground circuits (an exposed/broken wire that is touching a ground), short to hot (an exposed/broken wire that is touching a voltage source), parasitic draw, sensor signals, injector pulse, spark pulse, fuses, presence and quality of voltage, presence and quality of ground, resistance, diodes, and Hz/Duty Cycle. That's about all there is for electrical testing...they are all easy to learn how to do. The rest is just sitting down with a good diagram; learning how to best narrow down your choices; and finally actually getting to where you need to go sometimes can be tricky. It's not always easy, but you get creative. Heat, vibration, and sharp edges are the enemies of wires. Look for places with those things present first, and it's usually hard to go wrong.

I really hope that this helps. I'm not sure what kind of vehicle you have, but if it happens to be a W124 Mercedes I've got the wiring diagrams. I'd be happy to share. PM me if you need them.