Depends on your knowledge of electrics and what you want to do. If you don't know a whole lot and just want to understand things enough to fix simple stuff and know when it is time to call an electrician, get Don Casey's "Sailboat Electrics Simplified". That will get you off to a great start, and Nigel Calder's book mentioned above will keep you going.
Beyond that, Charlie Wing's book mentioned by colemj is fantastic. Some people are critical of him because he doesn't conform to ABYC standards, making John C. Payne's "Marine Electrical & Electronics
Bible" an excellent choice.
If you really think the electric
system in your boat just needs to be "maintained" (is there such a boat?
) then I would go with Don Casey's and Nigel Calder's books
. The other two are more for rewiring and if you wanted to add something somewhat complicated, like an isolation transformer.