Animorphs Book 35: The Proposal by K. A. Applegate

Marco is stressed out of his mind and it's affecting his morphs. His entire life is nightmare fuel and the nightmares are getting worse. This mission was interesting, simultaneously ridiculous in execution but very high-stakes.

Books like this one make me wonder about morphing technology. It's too bad that they (so far) can't be stressed-out in a controlled manner to create useful chimeras, but I suppose being very controlled can also produce this, like Cassie's mid-air morph in #34, The Prophecy.