It is not just the books. Please don't try to paint it now as just some dry lore dump in text, which would be poorly done worldbuilding. The story and lore of Genshin's world is woven into everything you see, hear, read, experience in the world. You just have to go with open eyes and an open mind through the world, otherwise you will miss a lot of details.
Up in the mountains, where some other traveller before you apparently took rest, you'll easily miss the fallen tree trunk next to a cliff, which is just perfectly placed to sit down, relax and enjoy the beautiful view. Not enjoying that view, you'll likely miss that when looking closer, and when applying some imagination, the golden flower fields of the valley below look like the scales of some giant snake lying in the valley and wrapping around the mountain. Whithout this awakening your curiosity, you are probably not curious enough to speak with the people in the nearby village about it. Without ever speaking to them, you'll never learn from one old man, that in legend this valley is supposed to be the resting place of a giant dragon, slain thousands of years ago by the God of this land, turned to stone and forming the valley and the mountain. If you never learned of that legend, it won't ring a bell when a character casually and factually mentions to you fighting a dragon up in the mountains to the north some time ago. When this doesn't raise any suspicions...and so on. You get the gist.
Genshin's world is packed with worldbuilding like this. Sometimes there's on top of that also history books, novels, fairy tales or even travel guides, really well written by the way, fleshing out the game world even more.