You can't. It's either an audio book, or a regular book. The controls have to be up on the screen for the audio book, so you couldn't have both that, and the text.
The only workaround is to put the audio book into mp3 files, and put them in the music folder in the order you want to hear them. They will play like background music, but you can't control it other than the stop/start controls for background music.