My nine-year-old has been plowing through these books at an alarming rate. His third grade teacher gave him a copy of book one to read over the holiday break, and, today, just over two months later, I ordered book 12. I would be into getting them from the library, the way he's going through them, but he keeps going back to the ones he's already read.
I suspect young girls will like these books quite a bit. There are wonderfully strong female characters, both good and not-so-good. My son also like some of the grossness inherent in the lovely rich details of how owls go about their business, which is far less messy than how seagulls do their business.
Soren is a wonderful protagonist, truly good, but not dreary. Your child will love these books and want them all. So save some money where you can. Buy the boxed set.