For theory I really enjoyed reading E.F. Codd's papers on the relational model. He was a good writer!
I am also writing a book you might be interested in https://agentultra.com/postgres-book.html where I focus on treating PostgreSQL as an application server. It covers data design, server programming, extensions, and integration with external processes and data stores.
The O'Reilly books are a good reference.
For theory I really enjoyed reading E.F. Codd's papers on the relational model. He was a good writer!
I am also writing a book you might be interested in https://agentultra.com/postgres-book.html where I focus on treating PostgreSQL as an application server. It covers data design, server programming, extensions, and integration with external processes and data stores.