Give Andres of Ebb his book, which you can find when he requests for it in Upper Calrath City. Make sure to speak to him every now and then before you progress Upper Calrath so he tells you about his book. From there, you can find it at an Umbral Scouring spot in a broken tower at the end of the Upper Calrath route. The tower requires you to climb a long ladder, and walk a pathway then climb down some spiral stairs till you reach a door. The door takes you back to Skyrest Bridge. Soul Flay the memory, and you will get the book.

From there, give him the book, and wait for all beacon bosses to be defeated. You can then progress the into Upper Calrath and get close to the arena with the Lightreaper fight. After the Lightreaper fight, you go through a corner to gert closer to the Upper Calrath City vestige, and you will learn more about Andreas of Ebb’s story arc.