The last two books seemed to me to have a slightly more adult tone. It was pretty clear KA assumed her audience had grown a bit since the earlier books. The villains were less cartoony-evil and more sadistic (Tom's Yeerk jeering about morphing Tom and beating Cassie, Visser One's killing spree), the violence was ramped up a few notches, the cursing was slightly more explicit (Tobias's "son-of-a-" comment), and the last book danced around the issue of sex a little.
It was a little jarring to see the Animorphs world take a step in that direction, not because the content was shocking, but because I could tell KA was taking the story past a few of the young audience filters that had been in place throughout the series. As much as I love the ending, I'd be really interested in KA continuing the story after shifting to an older audience.
