Prayer books are what you need if you want to expand the number of spells that vendors have available. Of course, these Prayer Books are not given to you immediately and need to be discovered. Search high and low Targeted one. This is where you find the Assassin’s Prayer Book in the Elden Ring.
After you’ve explored the game for a while, when you’re resting at the Site of Grace, you’ll be visited by Melina. She will take you to the Roundtable. This is the central area in the Elden Ring where you can meet NPCs and progress quests. You also have access to many of the vendors in this location.
When you get to the Roundtable Hold, you’ll be in a large room with a table in the middle and multiple exits. Go to the hallway door with the blacksmith. You will see a wide open door right next to him. Go over the threshold and down the stairs. At the bottom of the stairs, turn around and follow the hallway to the mist door with the imp statue.
You will need to use the 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’ll find the Assassin’s Prayer Book in a chest in the room.
https://www.gamepur.com/guides/where-to-find-the-assassins-prayerbook-in-elden-ring Where to Find the Assassin’s Prayer Book in the Elden Ring