It depends on what kind of book we're talking about. A lot of the fiction I read is just for entertainment then and there. Not retaining it isn't a big loss.

It depends on what kind of book we're talking about. A lot of the fiction I read is just for entertainment then and there. Not retaining it isn't a big loss. In fact, it is probably just good for me to forget. I read between 10 and 20 books per year that are worth retaining. The number of nonfiction books I read depends a lot on how much spare capacity I have for reading and absorbing them. They usually tend to take longer to read in total, and I often re-read chapters, make marginal notes, or take notes in notebooks. I don't often read my notes. I mainly take notes because note-taking itself helps me process and remember what I read. (In general I take a lot of notes and have notebooks going back a few decades). One thing that helps is actually exercising while listening to the audio book version. If I can listen to an audio book while hiking or bicycling, and I vary the route, I can often remember where I was when I heard something and thinking about a particular hike helps me remember what came next etc. The harder the physical exertion, the better I retain what I heard. (For instance, 8 years ago I was listening to the audio book version of an introduction to immunology and the chapter I remember the most of was the one I listened to while carrying my broken bicycle down a steep hill) Retention also depends a bit on how the book is written. Some authors write books that have low information density, which makes my mind wander. However, the real challenge isn't retaining the contents, but who wrote it and in which book. I remember ideas, arguments, and often quite a lot of detail - but I forget names and book titles. This is really inconvenient when recommending books I've read because I first have to figure out who wrote it or what the book's title was. (For work-related books I can usually check my notebooks).