DD read the first 5 Harry Potter books at home when she was 5 and 6, but I told her she'd have to wait until she was older before I'd let her have the rest. That lasted until she realized she could just check them out from the school library.
Anyhow, non-fiction -- we have a bunch of magazines as well as biographies, histories, science-related books, and... well, anything that seems interesting. DD will read pretty much anything that's left laying around. (When we moved, I put all the books not appropriate for <8 year olds in my bedroom to keep a closer eye on them.) You can start with things related to current interests: for Percy Jackson, I'd think anything about Greek mythology would be of interest.
