Nathan Ford I

The system seals the Will of an Etherna and bestowed it to humanity. 

When those who have the system complete some quest, they actually were fulfilling hidden requirements of being bestowed the will of a particular Etherna. This is the reason why Unique class or secret class have different prerequisite for each one, none is the same

The concept of the Book of Secrets is the same as the system. Nathan extract the Will that resides in his being from the moment he took the Unique class of Observer. 

And put it in a book. The book might have been just an ordinary notebook. But when the Will is sealed inside the Book, the book then become Fate-bound with him.

Fate-Bound and Life Bound items are different. For Life Bound items, it is usually ownerless, an item sometimes created by the accumulation of the energy and the will of the world.