Take no offense please, but "Poker For dummies" by Lou Krieger, is a fair starting point although published in the late 1990's so parts of it are dated as far as the state of online poker is concerned. Nothing that will hurt you it just that Limit holdem was the main game back then and Online poker was in its infancy.
Look up two Plus Two publishing. They have a beginners guide to poker that focuses on the basics of game mechanics, things like betting and raising and what hands beat what. This is little more then a pamphlet that was designed to be sold in a casino gift shop, With a little playing strategy thrown in. They also offer a lot of more advanced books. I think it is fairly cheap perhaps under $5.
A great way to get comfortable with the basics of the games is to play for free online. Most poker websites offer free games and you can learn enough to be dangerous. If you live near a casino or your on a trip to Las Vegas, you can just watch a game. They will be very friendly and encourage you to learn the game by actually playing with your own money great people they are.