Databases are something you will feel allowed to ignore if you work in frontend or in backend with an ORM; nevertheless sooner or later you will need to store your data somewhere. Internet is made up by databases in any kind of form to fulfill any kind of need. Without forgetting SQL in all its flavours (MySql, Postgres and MariaDb), this section aim to explore new alternatives used by modern companies to prototype faster or to store and deliver data in a more scalable and efficient way.
NoSQL Distilled: A Brief Guide to the Emerging World of Polyglot Persistence
Pramodkumar J. Sadalage
Data storage is a pillar of IT, but people tends to think that there is only a way to store data: RDBMS. indeed SQL, even is the main character on the scene, is not the only one aroud: key-value, documents, columns, graphs; there are dozen of way to collect and store informations and this book gives a good insight about them explaining the purposte of each one and what problem aims to solve.
MongoDB: The Definitive Guide: Powerful and Scalable Data Storage
MongoDB is probably the most famous NoSQL database; it solves natively some problems that are impossible (or at least quite hard) to face with classic E-R databases. This book is a good resource to keep at hand not only to understand how to work with MongoDB but how to administrate it too. A reference manual to deep dive into concepts like aggregation, sharding and clustering.