Animorphs Book 48: The Return by K. A. Applegate

Rachel suffers through an ordeal of psychological torment and has to re-make a decision she thought was settled. Through it she has a realization about her place within the Animorphs.

I appreciate Rachel's journey in this one. I think it's something that needed to be addressed before the end of the series. While the plot resolution is ambiguous, the realization she has and the character growth it belies is really important.