The Andalite Chronicles by K. A. Applegate

The Andalite Chronicles is the story of Aximili's brother, Elfangor, how he fought, hid, lived/loved, and returned as a hero. We learn more about Taxxons, Alloran's fate, and why Visser Three hates Elfangor. A solid prequel/midquel at the same time, don't miss it.

I'm appreciating on this read-through how the story threads pick up from the Hork-Bajir Chronicles to the Andalite Chronicles to create the conditions in the background when the whole Animorphs series starts. These should definitely be read where I have placed them if it's your first readthrough, but it is possible to read this as a prequel to the entire series if it's not your first time.

I really like Elfangor's story and how much it communicates by what he does and does not assume is normal. There really is a different perspective to the story when the narrators are not human, and this one is very well done.

As a mix between a prequel and a midquel, this is difficult to discuss without spoiling major revelations in the main series, but if you really don't care about series spoilers this could be read on its own as a self-contained (and really great) story.