This book was housed in the Restricted Section of the Hogwarts Library until Albus Dumbledore removed it in the 1950s. This was triggered by the student Tom Marvolo Riddle questioning his teachers about Horcruxes. It is assumed he read it in advance of questioning his teachers, as the book did not give any information on creating multiple horcruxes.
After this, Dumbledore removed it from the library and stored it in his office. In 1997, Hermione Granger managed to summon the book just before leaving Hogwarts at the end of her sixth year by shouting "Accio Horcrux books", thinking correctly that it would be worth a try to test her luck. Hermione took the book with her on the year-long quest to destroy Lord Voldemort's Horcruxes with Harry Potter and Ron Weasley.
It is a large volume, bound in faded black leather.