Animorphs Book 34: The Prophecy by K. A. Applegate

The Animorphs have an opportunity to help equip a Hork-Bajir insurgence. Cassie tests her control over morphing. Many of these books deal with loss, but this one has a lot of sudden shock & a very unique circumstance for mourning.

I really liked the opportunity to revisit the Hork-Bajir Planet. It's a really cool space, and even the description of it after the occupation is complete was moving. Because of the one-book nature of a particular character a lot had to be processed very quickly, but I think enough time is given for the emotional processing to feel realistic.