Many GMs prohibit the players from looking up rules mid-game. Not only does this facilitate smooth and timely play without bogging down in distractions, it removes the ability for players to spot loopholes and argue their way into ridiculously overpowered situations.
Players browsing the rulebooks during a game is just asking for trouble, really. Of course if you like that sort of trouble, there's nothing wrong with allowing it.
R2-D2: Sally, see what else is on the Stone Age weapons table.
C-3PO: Bows. 2d8 is enough to penetrate trooper armour.
[SFX]: shoot shoot
C-3PO: It says lassos and ropes can totally stop any moving creature.
C-3PO: Rocks, of course.
[SFX]: thunk! thunk!
C-3PO: Oooh, there's a trap section. Clashing logs. 20d6 damage.
C-3PO: And a logfall trap.
[SFX]: rumble rumble rumble
R2-D2: We should totally be using these against the star destroyers in the space battle.
GM: I'll have that rulebook back, thanks.