Silniel the Maven tells you that, irony notwithstanding, someone has asked the Thieves Guild to acquire a copy of a book called The Weight of Guilt. She tells you that she has checked the library in Firewatch three times and there's no copy there, but that she has heard Draan Solobar of Llothanis has a copy.
The only copy of the book in existence is the one Silniel has discovered, so your best bet is to head straight for Llothanis. The quickest way in real time is to head to Tel Ouada on boat, then take a River Strider to Llothanis, although if your patience and speed are both high it might be quicker in game time to make the long trek overland: Llothanis lies east southeast of Firewatch, past Ranyon-ruhn.
The book is on top of a pile on the shelf to the left as you enter Draan Solobar's house and can be taken using any of the standard theft tactics. Return with it to Silniel in exchange for 300 gold.
