"As I've said a few times, he didn't hide it, or even know where it was.
OK, from the top..
He was a mortal from your world.. Not really mortal anymore though.. Anyways, He got sick of all the meddling of the Gods, didn't like them using your people as Pawns in their games, and decided someone should do something about it.
The old man spent centuries learning about the Power, studying it slowly and carefully. He didn't go mad, managed not to destroy himself or the plane he was sequestered on. He tapped every archive in existence and created a few new ones. He was a genius in both magic and science.
After he gained the Power, he started using it to boot the gods around, one at a time. Eventually they grouped up, but they didn't wanna straight up fight, so they made a Deal.
The Old Man agreed to stop using the Power for anything but self defense, and let the book be hidden so nobody else could use it.
The Gods agreed to stay off the material plane, only affecting it through third parties - Summoned angels or devils, granting divine spells, that sort of thing. Also, that they would never look for or send agents to look for the book.
None of them even knew where it was - the old man and all the gods basically stood in a corner having drinks and watching each other while the heroes finished their quest.. "
"As far as where he is, whether he wants anything more now, whether you could challenge him, or who he really is..."
Wade shrugs "Beats me."