Just the player's book if you're strapped for cash. My friends and I played with just a player's book and we made up monsters and stuff (remember, we were fifth graders...) All the really important rules are in there and if you wanna use actual momsters there are tons of monster databases online. You don't have to keep the windows open if you don't like, just jot down the important info on a memo pad.

Edit: wait, baby stuff?