I've seen some negative reviews of Cassie's book due to her "personality change" and I can kind of see it. I've even seen some mention that, because of how subtle the change happens, it can be disorienting. Being a suspicious reader, I knew something strange was going on immediately, but I can see how it might throw some readers.
The change definitely raises the stakes of the book. One mystery is solved, but it's replaced by something more nefarious (and unlike the flowers, this effect is something the girls can't get away from).
Unfortunately, it also means Cassie spends a good portion of her own book being not right in the head, which means we don't really know how she would have handled her mission.
Still, a good continuation of the story and while her mission on Earth was not super exciting, the cliffhanger she comes back to is.