Baseball's not a very complicated game, but a cursory glance at the MLB Rulebook might lead one to believe otherwise. I'm glad that MLB makes their rules available online; I used to buy the hard copy version of it to peruse at home, and even now I'll refer to it every so often, but since I spend much of my time online anyway it's a lot easier to research the nuances of the game without leaving my mother's basement computer. That, and the fact that the omnipotent Google will let you search the whole thing makes it a lot easier to explore the details of even the most esoteric of baseball's rules.
The casual convenience of having MLB's rulebook online doesn't change the fact that it's often written like stereo instructions that were translated from English to Japanese and then back to English again. That's where the Baseball Field Guide (BFG) comes in handy. The book aspires to present baseball's often convoluted rules using plain but descriptive language and meaningful diagrams, both of which are sorely lacking from the official rulebook.
The Infield Fly Rule has sort of become the de facto "what's the deal" baseball rule over the years, even though it really isn't terribly difficult to understand and its significance to the game is perfectly logical and reasonable: prevent the defensive team from putting one over on the batting team. The BFG does a nice job of illustrating the scenarios in which the infield fly rule is called, as well as what would happen were there no such rule. The juxtaposition of the two clearly show *why* baseball needs the infield fly rule. The book's website provides an excerpt of the rule's presentation and is a good example of the book's style.
The one smallish gripe with the book, and I recall reading this in someone else's review, is that the index does not usefully highlight the primary entry for a particular topic. For instance, while it lists five page references to the infield fly rule, all but one of them are merely passing references. There's no way to tell -- without thumbing through each page listed, that is -- which reference is the main entry and which corresponds to an entirely different rule that happens to make mention of the topic you're looking for. It's a minor complaint, but it would make an otherwise tremendously useful book a bit more usable.
The average fan would find plenty of interesting items in here, and I suspect that any parent wishing to teach their kids the rules or little league coach looking to get a leg up would get a lot of use out of the Baseball Field Guide.
You can pick up a copy for about $11 at Amazon.