Sorceries are important if you want to be a potent magic-user in the Lands Between. While you can unlock new spells by finding them in the world, you sometimes need to go beyond that and find scrolls. There are three scrolls that you can find that each grant you access to new spells if you deliver them to Sorceress Sellen. The Conspectus Scroll will unlock the Glintstone Cometshard and Star Shower spells for you to purchase. Here is where you can find the Conspectus Scroll in Elden Ring.
The first step to obtaining this scroll is to go to Liurnia. This is the region to the north of Limgrave. You can get there by going through or around Stormveil Castle. Once there, you will need to get inside Raya Lucaria Academy. You can quickly get to the gate of the academy by using the warping stone in the Purified Ruins. There is a Site of Grace there that you can use for easy travel back to the gate. Go to the western side of the academy to find the Academy Glintstone Key. This item is found behind the slumbering dragon. Once you have the key, return to the gate and you will be granted access to the academy.
Once inside, follow the path through the area. As you make your way through the academy, you will find a large lift that is in constant motion. Use the lift to take you up to the ground above. When you get off, you will be near the Schoolhouse Classroom Site of Grace. From there, go through the doorway to the hallway and hang a left. You will see another doorway with a corpse in the next room with an item on it. Grab the item from the corpse to obtain the scroll.