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.
I read game manuals and strategy guides like my dad read the newspaper while eating breakfast on summer mornings. They had so much little bits of world building in the writing and design. Reading manuals on the ride home from the store set the tone before you even fired up the game for the first time
Games like Metal Gear Solid had mini comic book sections with loads of awesome artwork for their manuals
Manuals and guides are responsible for making me a gun nerd! Looking at the history and use cases for each one in the game carried over to the real deal. I remember guides for Rainbow Six, Medal of Honor: Frontline and Syphon Filter 2 being full of stuff like that