A picture book about a same-sex penguin couple at the Central Park Zoo was made inaccessible to young students in a Florida school district. The history of the Gay Pride flag was one of the most widely banned books in the country. An illustrated story about surviving slavery in America was prohibited in classrooms.

The number of books removed from schools and challenged in libraries has risen sharply over the past three years. Many of the books that receive the most attention are graphic novels written for teenagers or classics, like Toni Morrison’s “Beloved.” But picture books intended for young children have also been restricted, challenged and removed. Most of these books, according to the free speech organization PEN America, have protagonists who are L.G.B.T.Q. or people of color.

Those who push for restricting access to these titles say they are trying to protect young children from topics that they’re not ready to stumble upon while alone in a library, or that they’re too young to encounter at all. Those who believe these books should be available say it is crucial for young people to learn about characters different from themselves and to see their own lives reflected on the page.

Book restrictions have spread around the country in a variety of ways. Parents and organizations have created lists of titles they consider inappropriate or dangerous — lists often used as references by those who want to restrict certain books in their own communities. Individuals can work through their local school district to challenge one book, or multiple books at a time. In some states, such as Florida and Utah, new state laws have led to the removal of untold numbers of books.