I loved Percy Jackson, and you may be happy to know that the author has two new series in the works. One is also about he half-bloods and the first book is called The Lost Hero. The other is about Egyptian gods instead of Greek gods, and the first book is called The Red Pyramid. I devoured the latter in two days. The former is going to be released soon, if it's not out already.
The Sookie series is just great. It's funny as well (unlike the HBO series based on it, which is extremely vulgar and violent). It gets darker as they go along, but I just loved them. Be aware there is a bit of sex in them, which is not the case in any of the others you mentioned.
My daughter is reading the Guardian series and seems to like it. They both really are enjoying the series that starts with Magyk (Amazon.com: Magyk (Septimus Heap, Book 1) (9780060577315): Angie Sage, Mark Zug: Books: Reviews, Prices & more).
Based on what you said above, I'd recommend the Magyk one first.
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