I've never listened to a nonfiction audiobook yet that didn't benefit from bumping up the speed. With fiction, much depends on the reader. John Lee, for example, is excellent but a little draggy. Simon Vance, otoh, keeps things moving. I speed up the first; not the second.
Length of book also is a factor. Just to maintain my interest, I'm going to listen to virtually any book 20 hours or longer, whether fiction or nonfiction, at a faster clip.
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