DD9 has gone through the Harry Potter books at least a dozen times. She also went through a Rainbow Fairies obsession and one other series that I can't remember at the moment.

DD11 hasn't read a book at her level in years, mostly because those books just aren't as interesting to her. She enjoys young adult books like Percy Jackson or the Ranger's Apprentice (so do I, for that matter), even though they are well below her reading level. We've also focused on the love of reading rather than on making sure she reads challenging material.