( CNN ) -- Dav Pilkey 's `` Captain Underpants '' series was the most challenged book of 2013 for the second year in a row , according to the American Library Association . Pilkey , author of the illustrated adventures of two fourth-grade pranksters , was one of eight repeat authors on the list of most frequently challenged books in schools and libraries in 2013 . Each year , the association 's Office for Intellectual Freedom compiles a list of the top 10 most frequently challenged books as part of Banned Books Week , which runs from September 21 to September 27 . The goal of Banned Books Week is to celebrate the freedom to read and highlight the pitfalls of censorship . A challenge is defined as `` a formal , written complaint , filed with a library or school requesting that materials be removed because of content or appropriateness , '' according to the American Library Association . The number of challenges only reflects incidents reported , and the organization estimates that for every reported challenge , four or five go unreported . 40 must-read titles to feed your YA book addiction . The annual event started in 1982 , the same year the U.S. Supreme Court ruled that students ' First Amendment rights were violated when Kurt Vonnegut 's `` Slaughterhouse-Five '' and eight other books were removed from school libraries . Despite the legal precedent , schools and libraries still receive formal challenges to remove books from library shelves or nix them from reading lists to protect children from material that some see as inappropriate . Have you read any of these books ? Tell us in the comments what you think of efforts to ban them .
Dav Pilkey 's `` Captain Underpants '' series tops list of most challenged books again . Sherman Alexie , Toni Morrison and John Green are other repeat authors on top 10 list . `` A Bad Boy Can Be Good for a Girl '' by Tanya Lee Stone appears for first time on list . Comment: