Animorphs Book 14: The Unknown by K. A. Applegate

The Animorphs try to find out what's in Zone 91 and Cassie runs a race. It's a lighter story after the grimdark run of the first 13 books, a chance to breathe.

The Unknown is definitely a pause after a lot of setup and revelations in the start of the series. Most of the major players have been established and things are about to get complicated, but they haven't quite yet.