"The Hidden Game of Baseball" was written in 1984, one of the first truly analytic looks at the game and surprisingly, much that was revealed for the first time here is still relevant, and often not that well understood.
So for anyone whose familiarity with sabrmetrics (based on the name The Society for American Baseball Resarch, or SABR) begins and ends with "Moneyball," "The Hidden Game of Baseball" is an excellent starting point. The math is relatively easy to grasp, and though some of the formulas have changed dramatically, the concepts remain the same.
Today, unfortunately, much of the most advanced work is proprietary, developed and applied by Ivy Leaguers who work for baseball teams who have no desire to give up any advantage, no matter how small. Still, baseball fans who want to understand what's going on behind the obvious will find the curtain drawn back by the analysis in "The Hidden Game of Baseball."