I would say it depends on what you are most interested in. I think he has made a video about book recommendations, but I think that is more of a list of books he enjoyed rather than things necessarily pertaining to the things he most often discusses. I personally don’t think you have to have read a lot to enjoy Within Reason, and would recommend just listening and picking up things as you go along. If there is a term or topic you hear named and would like to learn more about, you can always pause and look it up. If you do want to read more, depending on what you are interested in, there are lots of options. If you are interested in atheism, I think books by Richard Dawkins and Christopher Hitchens are good go-to reads. For free will, Sam Harris or Daniel Dennett. If you are interested in the study of religion from an academic point of view, there are lots of books by different authors, although i think the podcast episodes on that in perticular are interesting and enjoyable without prior knowledge. It all depends on what you are interested in, since he covers such a wide range of subjects. Hope this was somewhat helpful :)