The Fifth Kingdom by Caridad Pineiro

Deanna Vasquez and her archaeologist mom had a falling out long ago when her mom went off in search of the elusive Montezuma's tomb. But when CIA agent Bill Santana pops into Deanna's classroom with the shocking news that her mom's been kidnapped, Deanna's ready to jump into action. Bill's on a mission to uncover the secrets hidden within her mom's latest discovery before the terrorists do and he needs Deanna's know-how to track it down. But there's a twist - Bill and Deanna have to get all cozy and pretend to be lovebirds in Mexico to make contact with the terrorists. And, as if that's not enough pressure, sparks fly between them along the way. Will they save Deanna's mom and keep the secrets safe? Only time will tell.