A good book series for a 13 year old to follow on after agatha christie?

i read the house of night series ages ago and i quite enjoyed them, they have both murder and romance in them and i thought they were really good.... twilight? maybe not everyones cup of tea but i did read them and enjoyed them but i hated the movies!!! Harry Potter, if you havent read them yet, you need to haha i loved loved loved them! the series of unfortunate event's were good, i only read the 1st two but my brother read afew more and he enjoyed them... Hope this helps :)