I’d start out with Moneyball by Michael Lewis. Lewis is such a great writer that it really adds to the “aura” of Bill James, Voros McCracken, and others.
The Numbers Game is also fantastic, but it’s a history more than anything else. I think Moneyball and The Numbers Game provide a good base of sabermetrics. If she really gets interested, the writing of Bill James, Baseball Prospectus, or any other sabermetricians (Dewan, Tango, Pete Palmer) is fantastic.
A really entertaining book is Baseball Between the Numbers. It is easy to follow, and introduces a lot of sabermetric topics. It’s by the BP team, so it’s very well done.