The first Cassie book. Yes. Cassie was always my favorite, and this book is a good one.*
The first four episodes of a TV show are usually used as an extended pilot: establishing the characters, conflicts, plot, etc. I think the first few books of the Animorphs series function in the same way. There are missions, but the kids are still figuring out morphing and coming to the decision that they are really going to fight the Yeerks. There's a pretty big reveal that solidifies most of their resolve (with the possible exception of Marco).
I found it interesting that Cassie is explicitly described as Black in the first book, but then there's no mention of her race again until this one. I can't remember if Marco has been described as Hispanic yet, but maybe KA Applegate was counting on his name and physical descriptions (dark hair, tanned/olive skin) to convey that.
Light spoilers under the cut.
*Dolphin morphs (acquired from Dolphins named after the Friends characters), meeting Ax (who didn't pay enough attention in school), the whale bits. This book is so weird it distracted me from wondering about their location and worrying that the Animorphs aren't being stealthy enough with their morphing/missions.