Tens of thousands of books are banned from US prisons, a new report said Wednesday, including a primer on drawing, a book about tying knots and textbooks teaching inmates foreign languages.

Prison authorities across the country cite vague "security" fears or worries over sexual content for their bans, which prevent some of America's 1.2 million inmates from reading often innocuous-sounding texts sent by friends, campaigners, publishers or bookstores.

A study by PEN America, a literacy and free expression advocacy group, found widely differing and often inconsistently enforced policies resulted in a "staggering" number of books never making it past a prison mailroom.

Victims of the censorship included "The Girl with the Lower Back Tattoo" a memoir by Amy Schumer, Sun Tzu's classic military manual "The Art of War," the century-old tract "The Use of Ropes and Tackle" and "Anyone Can Draw: Create Sensational Artwork in Easy Steps."

"Bans for purported sexual content were applied extremely broadly, from books on menopause to issues of Cosmopolitan and Rolling Stone, to art and medical books," the campaign group said in its report "Reading Between the Bars."

Only around half of America's 50 states keep a centralized list of banned books, with many others implementing ad-hoc systems often at a prison mailroom level, the group says.

Of those states that did know which books were banned, Florida was the most enthusiastic censor, refusing almost 23,000 titles to its prison population. Texas was second with around 10,000.

Reasons for the bans were varied, but the report found they were often very broad.

In Michigan, for example, "Spanish at a Glance" was not permitted because authorities believe it posed a "threat to the good order and security of the facility."

"(The book) may be used by prisoners to learn to communicate in a language that staff at the facility does not understand."

The pressure group communicated with inmates who said the rationale for banning the written word appeared wholly at odds with other aspects of prison life.

"Robert Blankenship, incarcerated in Virginia, notes that in Virginia prisons, the "Game of Thrones" novels are banned, but his prison airs the full, unedited HBO series on the facility televisions," the report said.

The group found the most censored title in the US is "Prison Ramen" -- a cookbook banned in 19 states offering recipes that can be made in a cell.

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