The old MS-DOS v4 manual (taught me about batch files, which was the first programming-like thing I did) and GW-BASIC manual that came with my 386 were the most important manuals I ever read.
I don't read game manuals. When I get a new game, I'm too impatient to read. I just jump right in, get-killed/lose/etc., and then learn from what went wrong. And then once I'm comfortable with the game, I read the manual to look up specific numbers, stats, etc., so that I could micromanage my gameplay.