How much time does she have to read in a day? Does she get to lose herself in a book at any other point in the day? My dd1 is very good about turning off the light and going to sleep as long as she has had enough time to read during the day, otherwise, she stays up late (which would not be a problem if she would learn to sleep in--but that is a separate issue).
I know that, for me, I will sacrifice sleep for reading, too. I love audio books for this purpose because they help me to wind down, and I can listen in the dark, which helps me to drift off faster.