The Leitners
In Episode 138, Architecture of Fear, Robert Smirke talks about having dreams of places, and speculated that they were the powers themselves. This is further proven by the fears becoming separate physical domains once the universe is shaped to fit them. In episode 80, The Librarian, Jurgen Leitner talks about the books being the fears' essences in a purer form, especially due to the fact that monsters and avatars n such have their own wills. Mikael Salesa says himself in episode 115, Taking Stock, that artefacts are not so volatile as the books. The seeds are even sown early in episode 17, The Boneturners Tale, that books have a power and are able to put your thoughts into another's mind. I posit that the books are more powerful and more potent fear artefacts due to their ability to put their "will", such as it is, into your mind. They take your mind and through imagination bring it into the domain of fear, even if it's unable to take your body. The power of the fears works through your mind and fear, though, which is why they are able to affect people's physicality.
Also completely unrelated but I made a post about jumanji a couple months ago and I'm really thinking it's just a little nugget of the hunt but it's such a pure artefact that it brings you to the Jumanji domain that has other powers in it