If you really don't know much and want a fun and easy read, I'd suggest reading Moneyball first if you haven't already read it. It gives you a good grasp of the basics (OBP, WHIP) while also showing you a bit if the battle between old school baseball guys and a new way of thinking. Almost everything in the book is obsolete in the sense that every team is using those stats in scouting, so they aren't as advantageous (is that even a word?) as they used to be. But its successful in giving you an idea of how advanced stats are used in picking diamonds from the rough and assessing player value differently.
Those who suggested Baseball Between the Numbers are right on. It will change the way you think about the game and does so in ways that keep you interested. Before I read it, I understood stats a bit but never really bought into how valuable advanced metrics could be. After reading the chapter on Bonds vs Ruth, it flipped a switch in the baseball cortex of my brain and made me realize how this stuff could meaningfully break down the game. For me, I was always interested in baseball history and arguing about players from different eras...knowing that you can create a baseline to compare players from a historical standpoint is what completely reeled me in.