Brown University’s library boasts an unusual anatomy book. Tanned and polished to a smooth golden brown, its cover looks and feels no different from any other fine leather.

But here’s its secret: the book is bound in human skin.

A number of prestigious libraries—including Harvard University’s—have such books in their collections. While the idea of making leather from human skin seems bizarre and cruel today, it was not uncommon in centuries past, said Laura Hartman, a rare book cataloger at the National Library of Medicine in Maryland and author of a paper on the subject.

An article from the St. Louis Post-Dispatch from the late 1800s “suggests that it was common, but it also indicates it wasn’t talked about in polite society,” Hartman said.

The best libraries then belonged to private collectors. Some were doctors who had access to skin from amputated parts and patients whose bodies were not claimed. They found human leather to be relatively cheap, durable and waterproof, Hartman said.

In other cases, wealthy bibliophiles may have acquired the skin from criminals who were executed, cadavers used in medical schools and people who died in the poor house, said Sam Streit, director of Brown’s John Hay Library.

The library has three books bound in human skin—the anatomy text and two 19th century editions of “The Dance of Death,” a medieval morality tale.