OK, you gobbled down all the Harry Potter books like Hagrid at an all-you-can-eat buffet for giants and now what can you read? The Waterboro Public Library blog answers that question in this blog post containing a list of Harry Potter "read-a-likes."
Librarians spend a fair amount of time helping patrons find books similar to books they have read and/or authors similar to authors they love and it's an art not a science. At BHPL we use the Novelist database to find fiction that might match a patron's tastes.
Here are some more links to fantasy reading lists for readers suffering from Harry Potter withdrawal. Of course these titles aren't the same, but you might find something...
Rockland Public Library
and from the American Library Association website there are several lists available.