A book probably isn't the best place to learn how things work. Taking apart a toaster, changing spark plugs on a car engine, or building a crystal radio is going to give you a far better sense of the inner workings of technological systems than words and pictures on a page. Books can be a good place to start, though, especially if you don't have the resources to hand your kids as many old appliances as it would take for them to teach themselves electrical engineering.
How many kids have gone on to become mechanics whose fathers forbade them from entering the garage? Or carpenters, whose parents didn't want them getting wood chips in their hair and clothes? How often does a couch potato learn to fly airplanes in virtue of her laziness? While there are exceptions of course, kids who aren't allowed to investigate it will generally end up ruled by technology, unable to comprehend its "mysteries."
Mechanical devices—from cars and trucks to microwaves and waffle irons that double as lawnmowers—fascinate children....that is, until we attach all kinds of limitations to them and snuff out any genuine interest. Half the reason we have a population of inept adults is that, increasingly, children are prevented from learning how to actually do anything. The best remedy for this uniquely modern malady is for parents to allow and guide their children in all kinds of practical endeavors, from working on the family car to building a bookcase to figuring out why the toaster no longer toasts.
But what do you do when you (the parent) don't know how to do those things yourself? While you could just dive in, a better option might be to get a book and start at the beginning. Our section is regrettably limited, but we're always looking to expand it with helpful and appealing titles—if you know of any, don't hesitate to send titles or series our way. In the meantime, we encourage you to teach your kids what you know, have a friend teach them practical skills beyond your ability, and just let them have a good time learning on their own, whether it's taking apart a small motor or putting it back together again.