The Tomes of Power are a type of Ashen Tome sellable as treasure to Bilge Rats which grant roughly 10 doubloons per tome and provide and unlockable for an Ashen cosmetic. The Tomes of Power are found by completing either a Ashen Chest Voyage or completing a Skeleton Fort in some manner to receive and Ashen Chest. These Ashen Chest require Ashen Keys to unlock, the loot contained within the chest are randomized but one of the tomes should be guaranteed.
The Sea of Thieves Gamepedia gives a rather basic rundown over the Tomes and how they can be acquired. https://seaofthieves.gamepedia.com/Ashen_Tomes