Reasons: Banned, challenged, and restricted for LGBTQIA+ content, and because it was considered to have sexually explicit images
Reasons: Banned and challenged for LGBTQIA+ content and because it was considered to be sexually explicit
Reasons: Banned and challenged for LGBTQIA+ content, profanity, and because it was considered to be sexually explicit
Reasons: Banned, challenged, and restricted for depictions of abuse and because it was considered to be sexually explicit
Reasons: Banned and challenged for profanity, violence, and because it was thought to promote an anti-police message and indoctrination of a social agenda
Reasons: Banned and challenged for profanity, sexual references and use of a derogatory term
Reasons: Banned and challenged because it was considered sexually explicit and degrading to women
Reasons: Banned and challenged because it depicts child sexual abuse and was considered sexually explicit
Reasons: Banned, challenged, relocated, and restricted for providing sexual education and LGBTQIA+ content.
Reasons: Banned and challenged for LGBTQIA+ content and because it was considered to be sexually explicit.
The Office of Intellectual Freedom, part of the American Library Association, tracks reports from libraries, schools, and the media on banned and challenged books. The 10 books above were the most challenged titles for 2021. If you'd like to see more of the most challenged books from past years, those are available on the American Library Association website: https://www.ala.org/advocacy/bbooks/frequentlychallengedbooks/top10. Lists are available for each year, or for the top 100 most challenged books by decade.
You can also view the books by category - Children's Books, Young Adult Books, Classics, or Books with Diverse Content
Some books that have been challenged or banned are available via Project Gutenberg. They are available on the Project Gutenberg website.
The Internet Archive also has a collection of banned or challenged books available electronically.