Shaman's tome can be found in the home of the shaman during the Legends quest. It doesn't give much useful information, more just the mental state of the shaman inside the flaming octagon and a mention of the fact he's actually being controlled by a demon.
After finding the book, another copy can be found on the shelves in your house.
You read the ancient shaman's tome. It is written in a strange sort of language but you manage a rough translation.
...scattered are my hopes that I will ever be released from this flaming Octagram, it is the only thing which will contain this beast within. Although its grip over me is weakened with magic, it is hopeless to know if a saviour would guess this. I am doomed...