The Tomes of Power, like the other Tomes currently in the game, are found in Ashen Chests, and must be unlocked with an Ashen Key. Ashen Keys can only be found be either killing an Ashen Key Master or fishing without bait. The easiest way we've found to guarantee yourself a key is to complete an active skeleton fort, but island hopping from small island to small island in the shores of plenty is also a good method. Once you have a key you need to either finish an ashen chest voyage (very easy) or if you did the fort you should get one from completing the fort to the reward room.
From there though it's all up to luck. At the time of writing there are a total of 15 Tomes, and the chests are randomized so you might get Tome of Power 1 and 5, but only get those for the next 20 or so chests. Keep at it, and just keep grinding away at it.
If you want more info on the Tomes here's a link to the wiki page: https://seaofthieves.gamepedia.com/Ashen_Tomes