The Prophecy — K.A. Applegate

There is a chance for the free Hork-Bajir on Earth to help start a rebellion against the Yeerks on the Hork-Bajir home world. And they've learned from the last surviving Arn--the race that created the Hork-bajir--that Aldrea, daughter of Prince Seerow, knew the location of a stolen Yeerk vessel packed with weapons.

Unfortunately, the only way to recover the ship is to ask Aldrea herself--and she's dead. But the Arn has Aldrea's persona stored. And Cassie, Rachel, or Toby Hamee are the closest match. The only problem is that once Aldrea gets into one of their minds, she may not want to come out....