Here’s a couple of examples Keep talking and nobody explodes — The most popular in the list I think
Uncle Chop’s rocket shop — the game where you are repairing your client’s rockets by following the in-game guidebook
Tin can — here you are also repairing the spaceshp but this time you are it’s capitan and you are in space in the middle of nowhere
If by “in-game manual” you mean one that’s part of the game world (not necessarily displayed on a computer screen) check out the classic Infocom adventures. For example, Border Zone came with a printed terrain map and tourist guide, and Sherlock and The Witness came with newspapers, among other things.
By in-game manual I meant a manual which built directly into the game and a main character could read it, not just a player who opened a PDF outside the game. And also usually those manuals are stylized, like in papers please (which I forgot to mention in the post btw) or like in Uncle Chop’s rocket shop