The Prophecy (Animorphs #34)

There is a chance for the free Hork-Bajir on Earth to start their own rebellion against the Yeerks. They have the numbers, but they don't have the weapons. 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. Unfortunately, the only way to recover the ship is to ask Aldrea herself -- and she's dead. The Arn have Aldrea's persona stored, but to bring her back, they need a host body, and Cassie, Rachel, or Toby Hamee are the closest matches. The only problem is that once Aldrea gets into one of their minds, she may not want to come out...