This is book 16 of the "Stone Barrington" series.

This note contains spoilers for the book.

Sent to Ft. Lauderdale to get a rich kid to sign a document, he eventually discovers (and has to convince Bill Eggers) that murders are happening and the kid is being hugely cheated.

Most of the murders end up being at the instigation of the cute girlfriend, who wants all the money for herself.