Megamorphs 3: Elfangor's Secret by K. A. Applegate

The Drode offers the Animorphs a chance to stop a Yeerk from changing history beyond recognition. They follow him through time, tracing hundreds of years of slaughter, sorrow, and gore. The tone swings between absurdity and horror.

All of the Animorphs react differently to the mission and to the cost proposed by the Drode. We see a stark example of how Cassie and Marco anticipate Jake's thinking, but in different ways. Because it switches frequently between perspectives we get the chance to see different reactions to nearly the same event. I appreciate how each of them has different ways of processing similar information, and also how they each pick up on different things around them. This book does a lot to develop Cassie, Marco, and Ax, in particular, as they tend to be more difficult to read in books which they do not narrate. Cassie is still dealing with events in The Departure, clearly not feeling absolved by her later actions in The Sickness. "Cassie, the killer with a conscience. Kill 'em, then cry over 'em."