Also, while we may see $75 or $150 or whatever as throwaway money (especially in the context of professional help products or books), that price is wildly unaffordable for most of the planet. Having high-quality free educational material out there is a godsend for students and developers in low income countries.
The Art of PostgreSQL is great and super thorough. Knowing the author I can't fault him on price for the effort he put into it.
But also your point of students and developers in low income countries completely understand that side. It's a shameless plug but we built our Postgres playground with tutorials on a number of topics which are completely free aiming to help target some of that audience - https://www.crunchydata.com/developers/tutorials
