THE IT GIRLS They're rich, fabulous... and dangerously underestimated. IT GIRL: Alexa Cheltingham MISSION: Pose as a grubby groom to stop a high-stakes horse killer and protect the Mayor of New York's show-jumping daughter. A Cheltingham shoveling horse manure — what would Mother think? That was my first reaction when the Gotham Rose spy ring handed me this riches-to-rags assignment. But the country's top show horses were dying suspiciously and the Roses trusted me to catch the killer. My old leg injury wouldn't stand in my way, not by a longshot. I'd muck stalls and uncover the dirty secrets of the equine world's bigwigs, then clean up in time to host the Horses of Hope charity ball. Speaking of the ball, I'd love to have charmer Ross Hardel for my date...too bad he's my prime suspect.