Lore can't be forced.
In most stories and games, a lot of lore goes untold and unheard. Tolkien had entire continents that shaped the world of Lord of the Rings, but while the effects of the history were seen, the actual stories were not (Aragorn's ancestors were originally lured to middle earth by Morgoth, first lord of Mordor, who created the balrogs!). Skyrim had entire libraries of books and dialogues about the dwemer, but most people who played the game were unaware of it. Dark Souls and Bloodborne have one of the most in-depth lore, but it's so broken up and hidden across the various item texts and dialogues that whole communities need to work on decoding everything, and most don't bother.
Having people care enough to read lore is one of the biggest challenges a world-builder can come up against.
So how do I make people care?
Now, you can go into the psychology of gaming and motivations, but the simple answer is "Tie your lore to what your players care about."
With DnD, the best ways to accomplish this comes from the three things that drive DMs crazy. Item hoarding, murderhoboing, and players hogging the spotlight. Old players or new, players have fun with cool items, cool battles, and getting the chance to show off their characters.
If you want to have your players care enough to read books, you might think about throwing down some pretty blatant hints. You find a broken sword that seems to radiate holy and unholy energy, and an identify spell reveals that, if fixed, this "ashkeeper's godslayer" sword is super powerful. Now the phrase "Ashkeeper" and "Godslayer" is tied to potentially getting some sweet loot.
Now they might read a page about ashkeepers, but a full book might still be too much. Now they face a horde of undead with some unusual ability, like zombies with truesight and who deal holy and unholy damage, and these are zombies of ashkeepers, and the symbol of the ashkeepers are all over their clothing. Suddenly, learning about ashkeepers not only might grant a cool sword, but also might tie into truesight, and maybe even some cool super zombies.
Finally, look into weaving ashkeepers into a character's backstory. Maybe one of your player's characters has a coat of arms, and the ashkeeper symbol appears in their crest. Now the players can have their character as the star of the week if they can push the ashkeepers as a central role.
And, by doing this, you can build up interest in the lore and also start sharing the lore. Just by what I've written, you know the ashkeepers wield holy and unholy powers, were formal enough to influence family coat of arms, they use enchanted swords, and are turned into zombies with extra powers. From there, you can build out the lore.
The best way to know if a party is ready to get heavier lore is if they begin actively looking for it. If they start looking through libraries for books with ashkeeper symbols, or go to a bookstore in a town looking for ashkeepers, or show off the broken sword and ask about ashkeepers.
What should I avoid?
In a lot of games, there's an obstacle I've heard called "paper fatigue". Players are constantly having to refer to their sheets and scan them for ability scores and modifiers, skills, feats, features, attack bonuses, etc. It's a lot of text, a lot of scanning, and it can be very frustrating and exhausting to go through binders every time you need to find a number. Adding more paper to this situation can be the straw that broke the camel's back, and you might lose party investment and immersion.
Also, avoid railroading. If the party doesn't want to read lore, don't punish them for it. If your players are more interested in running the story than digging into the history, don't punish them for it. You may need to find a different party.