I've read that excessive listening to music on headphones can cause hearing damage, but I listen to audiobooks and can do so at a very low volume and still enjoy them. I also wonder if it is as damaging because it isn't a steady stream of sound like music.
How long is it safe to listen to audiobooks on headphones?
I cover the answer to the vast majority of this question in this answer. Basically, there are standards regarding the total sound exposure that is safe, but the evidence base for these standards is limited. This a a nice online calculator for calculating the noise dose.
The unique part of your question relates to the difference between audiobooks and music. The safety standards are essentially based on the average sound level and the duration (there are special standards for impulsive noises like gun shots). With some relatively simple signal processing (phase randomization), it is possible to transform speech or music into noise without changing the average sound level. Therefore the standards and safe exposure limits are the same for speech, music, audiobooks and noise.
