Well, I wouldn't buy OOTP5 just for the manual, but you can always download the manual on its own. Much of what was in 5 is also in 6 in one form or another.
What I'd know before starting to play:
*The basics of the game of baseball. Balls, strikes, hits, outs - the nuts and bolts, so to speak.
*What makes a good player and what doesn't. Power, speed, defense, pitching control, velocity & movement, etc..
*A general feel for roles on a team - pitchers who start games, pitchers who come in later. What a pinch hitter, pinch runner, and designated hitter are used for.
*An understanding of why there are minor leagues and a rough idea of how to use them (as training grounds)
I think everything else is gravy, and can be turned on or off to make the game more or less complex.
