The Giant’s Prayerbook is a pretty late-game item which allows you to use Fire Giant Incantations, a sub-category of fire incantations, such as Giantsflame Take Thee and Burn, O Flame! In order to get it, make your way to the Guardians’ Garrison area in the Mountaintops of the Giants continent. Here, you’ll spot a tall building behind a Chief Guardian Arghanthy mini-boss; whether or not you choose to fight him is up you, but in the tower behind him there is a chest that contains the Giant’s Prayerbook.