Since somebody shared this nice comic about manuals in games in another community, I was thinking about them myself.
My most cherished game manual was the Diablo 2 one. The way they created a little story for each single ability was such an atmospheric wonder and probably started my fascination with lore instead of story. They were also probably the main reason why I took the necromancer and started to feel bored, when necromancer are automatically evil in a setting. Get creative!
My father had Falcon 4.0 and that was “just” a technical manual in itself. 5+ cm thick and full of schematics of the cockpit. I was in awe as a child about the complexity of that thing.
You guys should check out Tunic!
It’s a cute zeldaish adventure where a manual a a core part of the Gameplay loop. Basicly you find pages of the manual ingame as you progress and learn about it this way. And the manual looks freaking georgous. A pure love letter to exactly that time. But be beware that Tunic is a knowledge based game. So any information might spoil something for you. Please inform yourself with caution.
If you enjoyed the game you can even order the manual as a physical prop as official march.