Arsène Houssaye's book Des destinées de l’ame was added to Harvard's Houghton library in the 1930s by Dr. Ludovic Bouland a prominent medical doctor. Bouland had received the book from his friend Houssaye in the 1880s and because Bouland was quite the bibliophile he had the book bound with human flesh. The skin apparently came from the back of a mental patient who died from a stroke.
Bouland left a note in French tucked inside the book which when translated said:
“This book is bound in human skin parchment on which no ornament has been stamped to preserve its elegance. By looking carefully you easily distinguish the pores of the skin. A book about the human soul deserved to have a human covering: I had kept this piece of human skin taken from the back of a woman...”
Harvard knew they had the book in their collection but it wasn't until recently that the university actually got around to scientifically testing it.
To put the claims to the test Harvard scientists had to take microscopic samples from the binding and use proteins in the samples to identify whether they were human. After doing the analysis Harvard is now 99.9 per cent sure that the cover is human.

And while it may seem a little creepy, binding books with human skin (aka anthropodermic bibliopegy) was apparently quite common in centuries gone by and was often used as a way of remembering the person who died. It was also quite common for the skin of criminals who had been executed to be given away for book binding.
Des destinées de l’ame is now the only human bound book in Harvard's collection.... but that doesn't make it any less creepy.