The key to strumming is to make sure your right hand is moving almost constantly. I tell my students a lot of the time that your right hand is your drum pattern, and if you watch any acoustic guitar player they are moving their right hand most of the time. That does not mean that they are hitting the guitar necessarily all the time, but that fluid motion allows for the strumming to come more naturally. Like Jeff mentioned, really listen to the strumming and don't depend on books to aid you through this. You truly have to feel the strumming and allow your body to feel the rhythm you are playing and listening too. That is the best advice I can give.