Heart-shaped box / April Henry.

Enhanced descriptions from Syndetics:

The inimitable Claire Montrose is back, and as delightful as ever, in this third installment of April Henry's acclaimed mystery series. Claire wasn't part of the in-crowd at Minor High. In fact, she didn't have many friends at all, spending most of her time outside school tolling at Pietro's Pizza. Yet when the invitation to her twenty-year high school reunion comes in the mail, she decides to go.

After all, Claire is no longer the girl she was in high school, the one who was too smart, too tall, too skinny, and who looked a lot younger than she was.

Now maybe all her old handicaps are actually to her advantage. Not only that, she has a hip boyfriend named Dante, and a windfall from an old painting she inherited that turned out to be a Vermeer. So Claire sets off to see old friends, old lovers -- and maybe even a few old enemies.

While checking into Ye Olde Pioneer Village for the weekend festivities, Claire receives a mysterious package containing a heart-shaped box. Inside is a high school yearbook picture of Claire. A gift from a secret admirer? Then another box turns up -- in the limp hand of Cindy Sanchez, the former head cheerleader who is found strangled in the parking lot.

Before the night is over, five more women reveal that they, too, have received heart-shaped boxes. Is there a mad killer on the loose who will claim them all before the reunion is over? Or was Cindy, who had more lovers -- and more enemies --than anyone else i