I recommended “Understanding Distributed Systems: What every developer should know about large distributed applications” by Roberto Vitillo to all my colleagues back when I worked on SaaS systems.
“Designing Data-Intensive Applications: The Big Ideas Behind Reliable, Scalable, and Maintainable Systems” by Martin Kleppmann as the more advanced deep dive.
Both books provide timeless conceptual advice. Kleppmann’s description of developing a database by starting from an append-only text file really stuck with me.