[roundthecorner]

If you want a broad overview of how RDBMS work under the hood, "Designing Data Intensive Applications", would be a good conceptual start.

This could be followed by database internals books like "Expert Oracle Database Architecture" and "Pro SQL Server Internals", irrespective of whether you use these databases or not, you will learn a ton of stuff about database internals and system design.

If you are interested in database schema design etc, then look up "The Database Model Resource Book" Volumes 1,2,3. None of the above are quick reads but they are solid foundations for becoming a well rounded DBMS professional.