Edit: What's in the rule book? If you forget the rule book at home, can you get along or do you have to go back for it?

If there's no books at the table, it depends on whether your fellow players are willing to trust you to remember rules neutrally and if the DM is willing to adjudicate where no one can remember. There's also the online version for most of the core rules, although not all the exotic extra classes and stuff.