How to find Giant's Prayerbook - ELDEN RING
The Giant's Prayerbook contains two incantations:
Giantsflame Take Thee
Flame, Fall Upon Them
The Giant's Prayerbook can be found inside a chest at the very top of Guardian's Garrison in Mountaintops of the Giants.
The tricky part is to roll into some crates to reveal the broken wall in the garrison.