I'm currently learning back-end coming from a front-end career, and I started reading "Designing Data-Intensive Applications" by Martin Kleppmann, seems to be a must-have for anyone who wants to get serious in this field.
+1 For this recommendation, an exceptional book. In particular chapters 2-3 demystify how databases work, and provides an intuition on when to use different kinds of databases and query languages without the typical hype (spoiler alert - use an RDBMS!)