Prayerbooks are what you need if you want to expand the number of incantations that the vendors have available. Of course, these Prayerbooks aren’t given to you right away and need to be discovered. Search high and low Tarnished one. Here is where you find the Assassin’s Prayerbook in Elden Ring.
After you’ve explored the game for a while, when you rest at a Site of Grace, you will be visited by Melina. She will bring you to Roundtable Hold. This is the hub area in Elden Ring where you can come to mingle with NPCs and progress questlines. You also get access to numerous vendors in this location.
When you get to Roundtable Hold, you will be in a large room with a table in the middle of it and multiple exits. Head out the door to the hallway with the blacksmith. You will see an open doorway right next to him. Go through the doorway and down the stairs. At the bottom of the stairs, turn around and follow the hallway to the fog door with the imp statue.
You will need to use a Swordstone Key to unlock the fog door. On the other side, you will find another imp statue. This time, you will need two Swordstone Keys. Unlock the next door and you will find the Assassin’s Prayerbook in the chest in the room.