The Database Relational Model by C. J. Date

What it is about:

The core, rigorous concepts that underlie modern RDBMS technology with a clear derivation from first principles and from the historical evolution of the ideas.

Why it changed my thinking:

It made me truly understand what SQL is about and what databases really represent, how they operate and optimize and also very practical bits like how to handle NULLs, special values and "primary keys" in a way that is consistent with the underlying principles. It gave me the ability to jump across specific databases, ORM's and SQL dialects almost effortlessly.

Where to get it:

(Note, if you buy from the links here, I get a little commission back)