Another in the series of "Stake Your Destiny" books, in which Buffy and the gang must thwart Belakane, whose "Be the Ultimate You!" program is actually a cover for ... well, the cover should give you a clue. The alternate paths will keep you entertained for hours (at least, they did for me).
The stake your own destiny books take you back to the good old days of Buffy's high school years. But this time you're the slayer and you make the choices. Your choices effect who changes, if you live, and where you go.
Each story will be different, but the things that won't change are that first a strange woman shows up and claims to care about the student's self-esteem and the fact is just plain funny.
If you've never seen the show and still want to read them start with the book the harvest. But if that is confusing start with the movie tie-in. I recommend reading them in order if you've never seen it either.