This book nominally covers everything there is to do on a boat of an electrical or mechanical nature including diesel engines. But the vast majority of the book is on electrical equipment. After all, most of the things on the boat sooner or later wind up being part of the electrical system.
Many boats have two electrical systems in parallel. There's the basic DC system, usually 12 volt, sometimes 24. Then there's an entirely separate circuit for 115 volt AC power used when the boat is docked at appropriate places. This book covers it all. The first chapters are on the battery itself and the circuitry around the boat. Then it gets into battery chargers, inverters, wind and water generators, solar cells, everything you ever wanted to know. There are chapters on corrosion, lightning protection, radios and antennas and on and on. About half the book is on purely electrical devices.
The remaining half has chapters on engines, toilets, air conditioning and so on, but this also includes a lot of electrical devices such as autopilots, bow thrusters, and so on that may require electrical power.
Basically this is the bible on how to fix things on the boat. You'll save a fortune on repair bills, and even more if you're halfway across the ocean.