My DD8 has also loved Rick Riordan and the Warriors series. Warriors has 4 series of 6 books and then 5 super editions so they will last you a while. The series has some intense inter-feline situations (parricide, fratricide, lots of battles). It's not described graphically but some serious conflict happens between the cats. (I would say Warriors is not any more scary than, say, a queen asking a huntsman to hunt her teenage stepdaughter down and cut our her heart. Or similar. But that"s a whole other thread!)
Also, the Eragon series. DD devoured the series and the movie is fun to watch after.
