Like the Tomes of Power, and the Tomes of Curses before them, you can pull these books from Ashen Chests.
Ashen Chests can be found using maps that the Ashen Guardians drop, the keys to open them can be found from Ashen Keymasters. Both of these enemies will randomly appear at islands throughout the game, so just keep an eye out. They look like normal Skeleton Captains, with a red glow around them.
To make things a little simpler, if you find an Ashen Key, and don't have a chest. Head back to any Outpost and purchase an Ashen Chest Voyage from Duke, who is found in the tavern.
These cost 30 dubloons, but will give you a map to dig up a chest right there at the Outpost. Most of the items within then can be sold back to him, and you tend to break even, or come out a few coins ahead.
On my first chest, I pulled a Tome of Fire, so these can definitely drop from there. In the last update I got a Tome of Power (the new Time at the time) everytime I opened one, so it is possible the current new Tome is guaranteed everytime you open one of the voyage chests, but as I have only found one key so far, I cannot confirm this yet, but it is a working theory if anyone else can confirm.
Flamymex has pointed out he has opened many chest post update from Duke voyages, and has not always gotten a Tome of Power, in my experience, I opened 6 and got one everytime. So at the very least the chance seems higher, and it is certainly easier to buy a chest everytime you find a key, rather than scour the oceans lookiing for a chest.