The Book of Names contains every living person's three names.
The Book of Names was created by the Ancients for unknown reasons. It contains everybody's three names: your True Name, your Given Name and your Taken Name. Your True Name is the name that you are born with and it should never be spoken by another person. Your Given Name is the name that your parents give you and it can be used by mages against you to have limited control over you unless you have taken a name for yourself. Your Taken Name is a pseudonym that you get if you're a mage so that others cannot use you. When you are born, your given and true names are added to the book and the taken name is added later and states when or if you take one. If you do not take a name, it is not added to the book. The names are then erased when you die.
In the books
When it was found by the Sanctuary, the Elders approved of its destruction. However, they found no conventional weapons or magic would damage the Book. The Elders finally assumed it was only able to be destroyed by Ancients as it was made by them. The Elders then took it upon themselves to protect it from ill-fated hands. They secured it in the Sanctuary where anyone would have to clear the entire area before they can reclaim it. However that was not enough as guards can be killed and obstacles overcome so the Elders protected it with the Will of the Elders, a powerful spell which consists of a very simple defence mechanism: the closer you are to it, the less you want it, protecting it from wrongdoers (such as Serpine) as this book would mean infinite power for the person who owned it. It was held in the Sanctuary of Dublin but at the end of the first book, Stephanie caught a glimpse of her three names in the book moments before detective Skulduggery Pleasant used the book as a shield from the Sceptre of the Ancients. After Serpine accidentally struck the book with the Sceptre of the Ancients, The Book of Names was destroyed.
The Book of Names was seen as a pile of dust in a cage at the Irish Sanctuary.
In the alternate dimension, the Book of Names still exists. Former Grand Mage Eachan Meritorious is the only one who knows where it is hidden, but he is imprisoned by Mevolent. He is tortured frequently, but he refuses to let Mevolent find it, as it will allow Mevolent to learn his true name. As Mevolent could fight on par with Darquesse without knowing his true name, he would be essentially unstoppable. Its current location is only known by Meritorious but has not been found.