[jkingsbery]

Besides what others have mentioned (the entertainment value in reading), I think there are at least a couple other things at play.

1. When asked about a particular thing from a particular non-fiction book (or lecture, or podcast), one might not remember the text itself, but one may have internalized the point without remember where it's from. I know somewhere along the way I learned how to write a structured program, and there might be certain sources I remember being useful, but a lot of what I learned I couldn't off the top of my head point to a specific reference.

2. In a large amount of my reading, I don't read to absorb knowledge, I read to remember the reference for later. I don't need to try to absorb a lot of knowledge if I can easily pull up a book, web page, or document on my computer later.

3. Most facts I can recall off the top of my head are usually not things I read in a book, they are things I read in multiple books (or read in a book and later heard in a lecture or podcast). That doesn't mean that books don't work.

4. The style of the author can also come into play. Some authors are capable writers - they do their research and put together a coherent book - but some authors just excel at writing scenes that are memorable. David Mccullough was one such writer - compared to other non-fiction books I've read, I feel like I am able to remember more details from his writings than other similar writings. I think St. Augustine is also in this category - he spent a large portion of his life studying rhetoric, how to just keep people interested in what he was saying. Whether one agrees with Augustine, anecdotally most people I've talked to about his Confessions don't have the same problem as the author has in his discussions about The Selfish Gene. But I think writers that can write at that level are rare.