Inside a small town library in central Illinois, a book exploring the idea of separate races sat on the shelf. Rumor had it that Abraham Lincoln read that book, which flew in the face of everything he believed about slavery and human rights.

Illinois historians confirmed the rumor on Aug. 5 with the help of Lincoln's handwriting, which was scribbled on the inside cover of the 700-page book. With the discovery, historians offered reassurance that the future president likely read the book to better understand his opponents' thinking on the subject of race.

"Lincoln was worried that the whole idea that you could segregate one group of people based on some brand new thinking would just carry on into other realms," said James Cornelius, the Abraham Lincoln Presidential Library and Museum curator. "He could forsee the whole country coming apart over the issue that different people could be barred from different things based on different qualities."

Bobbi Perryman, the assistant director for the Vespasian Warner Public Library, didn't know whether to take the inscription seriously. The library eventually turned it over to the state historical museum this summer so officials could check the authenticity.

Cornelius and his colleagues quickly proved Lincoln had, in fact, read and written in the book. They made the assessment based on the look and spacing of the letters - the way he wrote his e's and n's. Lincoln didn't sign his note, but a local attorney confirmed underneath the inscription in 1861 that Lincoln had written it.

"There are certain letters of the alphabet that Lincoln wrote in a way that were not common to his era," Cornelius said. "A forger can typically do some of the letters in a good Lincolnian way. They'll give themselves away on a couple of the others. This all adds up."

Josiah Nott and George Gliddon wrote "Types of Mankind" in 1854 and circulated for decades at the Vespasian Warner Library in Clinton, Illinois. The book puts forth the theory that different races formed at different times and places and therefore they couldn't be equal, according to the Associated Press. Slave owners in the Civil War era used the text as a justification for their way of life, because the authors suggested enslaving Africans and Native American was part of the natural order.

The library is holding the book in a safe deposit box until it's restored and can find a safe place for its display.