What you experienced sounds like a form of hypnagogic imagery, which occurs in the transitional state between wakefulness and sleep. This is a common phenomenon, and many people report seeing vivid visual patterns, shapes, and scenes as they are falling asleep. The numbness in your body may indicate that you were entering a state of relaxation and your body was beginning to disengage from wakefulness.

The rapid eye movements you described, along with the vivid imagery, are also associated with the brain's activity as it shifts towards sleep. It's not unusual to be aware of external stimuli, such as your audiobook, during this phase, as you were likely still partially conscious.

This experience can be quite fascinating and even enjoyable, as you mentioned. It's a sign that your mind was entering a creative and relaxed state. If you find this state enjoyable, you might want to explore relaxation techniques or meditation to help facilitate similar experiences in the future. Just remember that it's entirely normal and part of the sleep process.