When you tap the screen to bring up the Audiobook controls for the first time, the playback speed setting will be represented by a 1X, or normal speed. Tapping that button will reset the speed to the 2X or faster speed. A second tap will get you 1/2X, which is of course the slower speed.
But, you may be wondering, why change the speed of your audiobook? Well, you might want to speed things along to finish the story before the end of a trip, or before the battery gives out. If it seems that the narrator is talking a bit fast, you might want to slow the playback down. Just be aware that the sound can become distorted when the playback speeds are changed.