@Sathorix as talysalankil already posted, your not supposed to enter the arcana obscura through the story, just go to lornars pass, then go to the hero point in the Durmand Priory. In this round room there are several gates below which all are closed. Talk to one of the durmand priorists standing next to gate to enter The Durmand Priory Instanz. I think its the gate just left when you come from the WP.
In the Instanz talk to Ogden Stonhealer and you will be able to enter the Arcana Obscura and read the book.
open world not story!