Tomes are obtainable via Ashen Chest which require an Ashen Key to open. The tome(s) you get from a single chest are random with the exception of tomes of resurrection that you can get for completing the Molten Sands skeleton fort during the February 2020 update. There are two ways about procuring any Ashen Chest and Key.
You can get chests from Skeleton Fleets or as a random drop from Meg or the Kraken, or some other things escaping my memory currently, and you can get keys from fishing, but I am concentrating on the ones you can farm with the smallest amount of waiting time between islands/events.Easy & Fast
If you have doubloons and/or gold to spare you can go to Duke in any of the taverns in the world and purchase an Ashen Chest Stash and Ashen Key Stash voyages. These will give you a X marks the spot map for the outpost you're currently located. Dig them up and role the RNG dice.Hard & Time Consuming
Check Duke's prices for yourself on the Stash voyages as Rare has been changing the price between updates as of this solution edit.
If you want/need to save your doubloons and/or gold then sail around the world looking for skeleton captains. These will be either Ashen Guardians or Ashen Key Masters.
Guardians drop a riddle order that you will have to complete to obtain the Ashen Chest while the Key Master drop an X marks order for the spot for the Ashen Key. The orders drop next to the Villainous Skull from the skeleton if you are having trouble locating it. Typically the orders are for the island you are located on.
Time Saving Tips
1. Ashen Guardians hold swords while Ashen Key Masters hold flintlock pistols. If you need keys because you have lots of chests (or vice versa) then this will let you utilize your spyglass to save you time from having to anchor or jumping off the boat to determine which Ashen captain you're looking at.
2. Ashen Guardians and Key Masters spawn in predetermined locations on every island. The complete list of these spawn locations for each island is a long list and this text heavy solution is enough of a read as it is, so I will give one as an example.
Cannon cove has a beach on the east side. The Ashen captain will spawn on this beach towards the south stretch of the beach.
2.5 This is something I noticed while doing the Thieves Haven Athena Run voyage and sitting on Thieves Haven while my crew sailed to the riddle islands for 7 hours one day. The longer you remain on an island the more locations on an island Ashen and regular captains will spawn on an island. The spawns are always close or on a beach.
3. Rare keeps changing things in the game (mostly for optimization and visual continuity between the Xbox console and PC) and at one time the Ashen Captains glowed red. Recently they seem to have removed this, or created a bug that removed this. So while you can use your spyglass to see what the skeleton on the shore is holding for a weapon, it might be a named captain or an ashen. The red glow could return in a future update.
If you give any TA solution a down vote please leave a comment as to why. Solutions cannot be improved without feedback.