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.
Not exactly a manual but I got stuck in Day of the Tentacle. Finally I caved and ordered the official hint book. Back then that meant sending them a letter. No idea how payment worked. My parents probably helped me with that.
It took an eternity to arrive. A few weeks at least. In the meantime I tried to progress the game. The day before the book arrived I managed to do it (use physics book with horse). And once I was past that I managed to beat the rest of the game.
But it wasn’t in vain. The book contained the story of the game written like an essay by Bernard (basically the game’s main character). It was pretty funny. So I didn’t regret ordering it.