Animorphs Book 33: The Illusion by K. A. Applegate

Tobias volunteers to be captured in order to trick the Yeerks, knowing he might be tortured. He accepts the unsettled nature of his existence and gains a piece of his heritage. This closes the several-book AMR arc for the team.

This book contains depictions of psychological torture, extended sequences of it. It's a tricky bit of world-building, because the nature of the torture involves using painful memories alternating with pleasant memories, most of which haven't been specifically shown in the books. It puts more detail into Tobias's backstory without being sudden revelations, and I think it manages the balance well.