Like all activities, it's trial and error. I can relate to your experience as I've had a similar path over the years. Audiobooks are useful to enhance the audio memory. While reading printed pages will enhance the visual memory, I think it's helpful to develop the memory in more than one dimension. With audio I like the fact that I can speed it up or slow it down to match my state and current pace. I really think the brain is wired in such a way to absorb information by listening. If I were you I'd notice the kind of books that make you sleepy. Since info can be absorbed on many levels, the subconscious knows things on a deeper and more meaningful way. Sleep is when the data gets compounded and other processes occur inside and outside the body to make sense of what's what.