On steam, the Holder of History achievement is simply described as “Unlock all lore”. Lore tablets and lost messages are completely new items in the game, and can be obtained in various ways throughout your crusade.

The Sins of the Flesh Guide & Help post states the following, which can give us a clue as to where to find the tablets, this will be the basis for this guide;
“You need to have defeated The One who Waits to unlock the Lore Tablets. Find secret lore totems underneath breakables during crusades.
There are 15 Lore items: 10 that can be found in the lore room, 3 in the new graveyard room by digging up graves, one hidden behind Haro’s tent or in the Meat Room and one hidden behind the fire in Monch's room.”

any additional artwork not from the game is created by me
credit to the Cult of the Lamb fandom wiki [cult-of-the-lamb.fandom.com] for sprite images and some item descriptions

10 lore pieces

These “special” rooms can be found randomly in any combat dungeons, the biome/environment doesn't matter. These rooms are hidden from the player and need to be unlocked by interacting with statues in combat rooms.

Hidden rooms are signposted by unique crown statues that will appear in combat rooms, if a hidden room is present there will always be two statues, one inside the walkable area, and another outside the edge of the room which will mark the path to the hidden room. In order to uncover these, it's best practice to clear out any rocks or other environment props that might obscure them in each room, this shouldn't take you long and ensures you don’t miss them, as well as granting resources for clearing space :)

The easiest way to find these rooms is to equip the Telescope tarot card to reveal the map, this will allow you to see when hidden rooms are present where you may otherwise miss them, as their entrances need to be unlocked and could be easily missed. On the map, they will be marked with a scroll/paper icon.

A hidden room will always be north of it's connecting room, so if there isn't any space on the map to allow for that you can rule out the possibility and continue the search elsewhere!

The statue in the room, the doorway once unlocked

To enter the room, attack the statue in your combat room, it may take a few hits to activate. When successful, a red beam will link the two statues together and destroy them, clearing the pathway to enter.

When you enter, you will walk to the centre of the room and need to sacrifice a red heart to receive a lore tablet in return. You can then continue with your crusade as normal!

There should be 10 of these in total, the rooms are seemingly completely random so just keep crusading and exploring until you get them all.
Once you have obtained all 10 tablets you will also obtain the snake follower form.
The inside of the hidden room and sacrifice altar

3 lore pieces
Dungeon graveyards are a type of dungeon that can be encountered in “?” locations on the crusade map, they consist of four or five graves that you are prompted to dig up. Lost messages can be found here when digging up one of the graves (these graves also have a chance to spawn enemies, so be careful!)
The graveyard room, a lore piece can be buried in any of the graves

2 lore pieces
Two additional lore pieces can be found in npc/resource rooms;
Monch can be encountered in any "?" room, and will raise the loyalty of one of your followers. However if you walk behind his bonfire you can see the "pick up" prompt appear, if you do so you will obtain a Lore note.

"Pick up" prompt behind Monch's campfire

Haro is encountered at multiple scripted points throughout the game, however during the story he can also be encountered randomly on crusades. You can find a Lore note after talking to him in these rooms.

Haro's random encounter room

(Thank you to Nietbern for informing me about Monch and Haro's rooms!)

Meat is one of the potential resource rooms you can encounter on crusades, it will show as an icon on the crusade map. If you cannot find Haro then visiting this room and destroying the chunks of meat in this room can drop a lost message.

(Thank you to Shisei11 and ScaryHockeyGuy for informing me about this!)

The meat resource room

Note that you only need to visit Haro or the meat room, both provide the same lore piece so you only need to find one of them.

I really appreciate all the support in the comments here helping me complete this guide and figure stuff out! Now that an official guide has been released I would always reccomend checking that out if youre uncertain about other features of this update, but I hope that my guide can still be useful in adding detail to some of the tasks. I'll update this if anything changes, but all in all this should be enough to help you get the achievement (it worked for me!)

Thanks again for taking the time to read/contribute, and I wish everyone luck in uncovering all the lore!