Just about anything written by C. J. Date would be worth your while in this regard. It will give you a solid grounding in the relational model, which you can then apply to any RDBMS. Here's a good list to start with:

- Database in Depth, O'Reily (2005)

- Relational Theory for Computer Professionals, O'Reilly (2013)

- SQL and Relational Theory, 3rd Ed, O'Reily (2015)

- Database Design and Relational Theory, 2nd Ed, Apress (2019)