The PostgreSQL manual [1] is an incredible resource. The first 100 or so pages give you a lay of the land. If you read those you will be lightyears ahead of the average web dev. After that you can dig into other chapters/topics that interest you.
I always appreciated that Postgresql documents their differences from standard SQL. When I want to know the standard way I often revisit their documents, in addition to those of the database I'm using.
Even though I've never sat down and read it, I can say with confidence that the Postgres manual is one of the most impressive manuals I've ever seen. They could have published the manual as 2 books, and I would have paid money to read them.
[1] https://www.postgresql.org/docs/manuals/