Databases are the pragmatic intersection and application of all the fundamental topics in CS. Algorithms and data structures, distributed systems, PL and compilers, operating systems, hardware, etc.

If you're a developer and you haven't read Designing Data Intensive Applications, treat yourself. It is an excellent introduction to data and distributed systems thinking, and it's one of the best written and most fun books on software. For experienced developers, there is no other book I know of that packs a punch like this one. I do not make any suggestions for any other software books until you say you've read this one. And disregard the bits about scale, it is relevant for any developer that stores or passes along data between more than one system. That is, it is relevant for basically every developer.