Animorphs Book 8: The Alien by K. A. Applegate

The Alien deals with isolation, uncertainty, and learning when/whether to trust. Ax must decide whether he's one of the Animorphs or just conveniently aligned. Ax tells the humans how Seerow's Kindness doomed their planet.

This book stands out for being the first real look at how differently Ax approaches everything as an Andalite. Its treatment of isolation while surrounded by friends and allies manages to be insightful without just treating Ax badly for not embracing the Animorphs as completely as they welcomed him. It's a delicate balance because too far in one direction would make it harder to trust them as a unit later in the series, but making him trust them too easily would make the conflict in this particular book seem pointless. It helps that there is a specific reason that Ax is hesitant to trust that goes deeper than just being uncomfortable, and I think it's handled well here.