From Granblue Fantasy Wiki
Tomes are primarily used to uncap characters. They are needed for early stars on R and SR characters.
|Uncap R to 1★||2|
|Uncap R to 2★||3|
|Uncap story SR to 1★||2|
|Uncap SR to 1★||2|
|Forging Class Champion Weapons||70|
|Upgrade Revenant Weapons (step 1)||100|
|Upgrade Revenant Weapons (step 2)||150|
|Buy 1 Scroll from the Treasure shop||5|
|Buy 1 Anti Verse from the Defense Order shop||25|
Tomes are found in the Cerulean Trial daily quest and are rarely found in event quests.