Not all books are spinach.

From The Writer's Almanac:

On this date in 1966, the Vatican abolished the Index of Prohibited Books. Officially known as the Index Librorum Prohibitorum, the Index was made up of books that Roman Catholics were forbidden to read for fear of endangering their faith or their morals. The first formal index, called the Pauline Index, was published in 1559 by Pope Paul IV, but the church’s practice of censoring or forbidding books had been going on for over a thousand years by that time. In 496, Pope Gelasius I had put out a list recommending certain books and banning others, but since books were produced in limited numbers and kept in private libraries, it wasn’t a pressing concern. Most people would never have access to any of the books on the list. But Gutenberg’s printing press and the invention of moveable type in the mid-1400s meant that books — and the ideas they contained — could quickly spread all over Europe, and could fall into nearly anyone’s hands. This was fine when it came to printed Bibles or pro-government propaganda, but alarming in the case of dangerous new scientific theories or political dissent. So the church and the government began to try to regulate who could print and publish books.

With the publication of the Pauline Index in 1559, the works of 550 different authors were banned. If an author only wrote one heretical book, or if he was a Protestant, all his works might be forbidden, because his moral corruption was considered to infect everything he wrote. Many of the books by Protestant scholars had nothing to do with religious dogma, but even their works of botany, law, medicine, geography, and other sciences had to obtain special dispensation to be printed, or were banned outright. Roman Catholic authors were sometimes given the opportunity to edit their works to make them acceptable for publication.

Some of the authors who found their names on the Index at one time or another include astronomers and physicists Nicolaus Copernicus, Galileo Galilei, Giordano Bruno, and Johannes Kepler; philosophers John Locke, Thomas Hobbes, Immanuel Kant, and Jean Paul Sartre; and authors Jonathan Swift, Victor Hugo, Gustave Flaubert, and Graham Greene. Uncle Tom’s Cabin was considered for inclusion, because some thought it was a veiled call for revolution, but it was ultimately left off. None of Karl Marx’s work made the list, nor did anything written by Adolph Hitler, James Joyce, D.H. Lawrence, or Charles Darwin.