Yeah, the miller literally makes you eat a bit of a shit covered corpse for a jewish golem ritual and this is the second part of it you can continue after you get into the castle and crash the wedding. The book he was looking for was in the castle, hence how he was able to sneak you in there. This quest also introduces a girlboss gypsy who apparently traveled all over Europe and became an assassin or something, somebody's self insert who probably should have been the main character instead of Henry.