Are the books preferred and most enjoyed by children harder than other books they read? Are non-fiction books read and understood at the same level of difficulty as fiction books? The Accelerated Reader software offers computerized comprehension quizzes of real books individually chosen by children, giving children (and teachers, librarians, and parents) feedback on performance. Children can also express preferences for books they enjoy. Quizzes taken by 150,220 children in 967 schools are reported. Considering highly preferred books, children were reading very difficult books with a high degree of success. Non-fiction books were not read or understood as carefully as fiction books. Implications for future research and practice are outlined.