I remember when I got Falcon 3.0. The manual was 250+ pages [1]. Unfortunately for me, my 16MHz 386 just couldn't get the job done. I thought it might be playable with cranked-down details, but it really wasn't. So the manual stoked my imagination for an incredible flight sim on a capable computer.

[1] http://lucasabandonware.free.fr/manuels/Falcon%203.0.PDF

I personally was a big fan of 688i Hunter-Killer. One of the most realistic sub games ever made, with a 250 page manual that frankly under-explained how a lot of the more technical systems on your boat functioned (I'm still not quite sure when it's a good idea to let out the port/starboard towed arrays!). To get good at the game basically required you to take a crash course in modern submarine warfare.

http://www.stormpowered.com/stormcloud/games/6040/manual.pdf

Man, I really miss these kinds of games. DCS is really all that's left of what used to be an entire genre.

The original manual was about 200 pages! I can’t find it anywhere online. It goes into tons of other things — like the construction of this plane and dogfighting tactics (even though this plane isn’t designed for it) :)

I had one for the game about the Apache attack helicopter. When you landed after the mission the base asked you a "challenge" which required you to respond correctly or you would be shot down. This was probably a kind of copy protection. The correct response was printed on each page in the manual, one challenge-response per each page!

Yeah, definitely copy protection, as "type in something from page X of the manual" was a standard method these days; but it seems they've managed to fit it nicely in the theme of the game, too.