I'll second the praise for the Postgres documentation. I learned a lot about Postgres simply by looking through the documentation and reading the parts that looked interesting.
For example https://www.postgresql.org/docs/13/wal-configuration.html is done properly: it describes what that part of the program actually does, with the settings that affect it introduced at the appropriate point, separately from the reference documentation which lists the same settings in alphabetical order.
But not all aspects of Postgres get this quality of documentation. It would be good to see a similar treatment of the checks the planner or executor performs to decide whether to permit access to a particular table and column and row, rather than having to deduce it from the separate descriptions of the security features.
(For example, the basic fact that a VIEW accesses a table using the privileges of the view's owner, not those of the user executing the query, is very hard to find in the documentation.)
Interestingly enough the competition for "biggest impact on my SQL Knowledge"... Is a book about MS Access 97 which taught me enough about how SQL actually works that I could understand Postgres docs.