The manual is a booklet that comes with the games and contains lots of information about the game, such as how to play, tips and general information.
A pdf of the Dungeon Keeper Manual can be found here. Please note that it contains some errors, likely due to changed or unimplemented plans for the game.
The Dungeon Keeper Gold Manual can be found here.
GOG version here.
Dungeon Keeper 2Edit
The Dungeon Keeper 2 Manual can be found here.