Well, it depends.
When creating characters or NPCs, I usually take a quick look at relavent books if I have the time. However, I usually do this ahead of time because I can usually remember a feat or two, or have the effect written down. I can build a character with Dragon's Toughness, but I don't need to have MotW around.
If I'm playing a spell caster, I usually bring lots of different books because they can have a wide variety of spells.