In academic books' case, there is a parasite in between the author and reader: the publisher. Which we know they tends to charge exorbitant price for copyediting or digitizing by low-cost labor. The authors receive only a tiny fraction of the sale if not nothing. I will do whatever I can to help the parasites die.

For non-academic books, I don't know how things works, maybe there is a mechanism where the author can gain revenue from selling books. So I always pay for them and the priated copy I downloaded is just for archival.