tl;dr Derek sells book content and media separately: content: $15 (one-off) digital: $0 print: $4/copy Comment #1 on the post was exactly what I was thinking:

tl;dr Derek sells book content and media separately: content: $15 (one-off) digital: $0 print: $4/copy Comment #1 on the post was exactly what I was thinking: "Interesting. My sense is that when people are buying paper books for their friends, then the content is new to each of the friends, and it's fair to charge another $15 each time as well as the $4 for the paper. But if I want a spare copy for myself, or maybe a replacement, then it seems fair to only pay $4 for the paper."