I typically have more than one book 'on the go'. I'll have one on my Android phone, an audio book running in the car, and another on the phone just in case and I usually have three or four books running on my ereader. The only rule that I apply is that I try and avoid similar books at the same time. So one could be be a SciFi, another Horror, another Biography, etc. And thats only so that I don't lose track and get confused with similar situations arising.
