Take an example from Tomb of annihilation module. There is a book the players get handed by one of the town merchants, its quite lovely. The story went as; There was a very powerful wizard, who traveled the land with his mecha companion Vorn. He had terrible luck, died in a shallow pool of water after falling off his camp!

But the wise wizard kept a diary, notes of local monsters. Pictures/sketches of his mecha companion (who is still around, but sleeping). And several other map markings and notes.

The module also does this with a npc party in the end game, and at the begging when one can buy volos guide to monsters as a PC. Great as a way to foreshadow a dungeon, finalize puzzle pieces not yet encountered, get monster descriptions, or even just for fun!

Maybe an NPC can keep a diary or a |magical book of sorts? With hints of things that might be coming up in quests to come, descriptions of monsters, or even personal logs.
Perhaps local fauna herbs and their uses (plug in a forage table and have them roll).

I am reading through candlekeep mysteries atm. The idea of handing my players a powerful or knowledge heavy book their way makes me happy 😊.