The Ultimate (Animorphs #50)

Really big trouble is an understatement these days. The war with the Yeerks is full on -- and it's definitely going to get worse. But Cassie and the other Animorphs have a lot more going on than just trying to stay alive. Now they've got to actively protect their families and the free Hork-Bajir. And they no longer believe they can do it alone. The Yeerks are just too powerful.

So Cassie and the others have to ask themselves a very important question: Is it time to increase their numbers? They all remember what happened with David -- the Animorph gone bad. But this time do they really have a choice?