Dav Pilkey's children's book "Captain Underpants" and E.L. James' erotic novel "Fifty Shades of Grey" top US Library Association's Annual 'Challenged Books' List.

Every year the US Library Association releases an annual list of the most "challenged books." For this year, Dav Pilkey's children's book "Captain Underpants" took top position while E.L. James' erotic novel, "Fifty Shades of Grey" came in at number four. The listing is based on complaints that the association receives from parents, teachers and other members of the public. Most of the complaints are objections regarding offensive language used in the books as well as graphic sexual content.

Sherman Alexie's prize-winning "The Absolutely True Diary of a Part-Time Indian" came in at number two after the association received complains about the book with regards to offensive language used in the book as well as it book being racist and sexually explicit. At number three was Jay Asher's "Thirteen Reasons Why," which made it to the list owing to complains about the description of drugs/alcohol/smoking along with it being sexually explicit and about suicide.

"It's pretty exciting to be on a list that frequently features Mark Twain, Harper Lee, and Maya Angelou," Pilkey said in a statement. "But I worry that some parents might see this list and discourage their kids from reading Captain Underpants, even though they have not had a chance to read the books themselves."

According to The library association's Office for Intellectual Freedom, a challenge is deemed as a "formal, written complaint filed with a library or school requesting that a book or other material be restricted or removed because of its content or appropriateness."

This year, the association received 464 challenges, which was 25 percent more than it received in the previous year. However, it is still lower than the number of challenges the book received in the 1980s and 1990s. The association has long believed that for every complaint registered, four or five go unreported by libraries, and that some librarians may restrict access in anticipation of objections.