I read game manuals all the time.
The fact that they're getting smaller can be both good and bad, but I see nothing wrong with a manual that is thick. I still have the enormous manuals from Hexen II, Roller-Coaster Tycoon, Civ IV Gold, and both Diablo titles, and I still flip through them at times.
Actually, here's what game companies, especially RPG developers, should do: Get inspiration from the Gold Box games and make manuals a part of the experience of the game. If you look back at the classic TSR Gold Box games, they had a book included that contained background information and plot hints for the game.
And yeah, I know that was to save disk space since those games used 5-1/4 or 3-1/2 floppies, but in a way, I think this could make manuals more useful, and act as an anti-piracy method.