Databases
MySQL
Efficient MySQL Performance by Daniel Nichter
High Performance MySQL by Silvia Botros & Jeremy Tinley
Redis
Redis Data Structures for Non-Redis Users
General
Seven Databases in Seven Weeks by Luc Perkins
Beginning Database Design: From Novice to Professional by Clare Churcher
System Architecture
Designing Data-Intensive Applications by Martin Kleppmann
Microservices Antipatterns and Pitfalls by Mark Richards
Concurrency
Grokking concurrency by Kiril Bobrov
C++ Concurrency in Action by Antony Williams