The American Library Association in their State of American Libraries report for 2013, listed the Top Ten Frequently Challenged Books.  These numbers from from 464 recorded challenges to books that year:

The Adventures of Captain Underpants (Captain Underpants, #1) The Absolutely True Diary of a Part-Time Indian Thirteen Reasons Why

Captain Underpants (series), by Dav Pilkey (offensive language, unsuited for age group)

The Absolutely True Diary of a Part-Time Indian,” by Sherman Alexie (offensive language, racism, sexually explicit, unsuited for age group)

Thirteen Reasons Why,” by Jay Asher (drugs/alcohol/smoking, sexually explicit, suicide, unsuited for age group)

Fifty Shades of Grey (Fifty Shades, #1) And Tango Makes Three

Fifty Shades of Grey,” by E. L. James (offensive language, sexually explicit)

And Tango Makes Three,” by Justin Richardson and Peter Parnell (homosexuality, unsuited for age group)

The Kite Runner Looking for Alaska Scary Stories to Tell in the Dark (Scary Stories #1)

The Kite Runner,” by Khaled Hosseini (homosexuality, offensive language, religious viewpoint, sexually explicit)

Looking for Alaska,” by John Green (offensive language, sexually explicit, unsuited for age group)

Scary Stories (series), by Alvin Schwartz (unsuited for age group, violence)

The Glass Castle Beloved

The Glass Castle,” by Jeannette Walls (offensive language, sexually explicit)

Beloved,” by Toni Morrison (sexually explicit, religious viewpoint, violence)