The Stranger (Animorphs Book 7)
The last time Rachel and her friends entered the giant Yeerk pool beneath their town things went very bad. This time they plan to be careful, and sneak in as roaches, just to spy.
Once they're inside, though, the team gets caught. But right as things are looking their most dire, everything stops. Everything.
Then Rachel and the Animorphs hear a voice. It belongs to a very old, very powerful being, and it says it can save them. But if it does, the Earth will be defenseless.