Two books by Uncle Bob, ‘Clean Code’ is on coding techniques, where ‘The Clean Coder’ is about professionalism and soft skills. Both are excellent books.
Relational Database Theory:
A great introduction to the relational model, relational operators and database design. CJ Date has a lot of books available on this topic, but this one would be a good starting point.
The ‘Deep Dives’ are collections of writings by MVPs and other notable people in the SQL Server community, covering Admin, Development, BI and other database topics.
SQL Server – Design:
Database Design for Mere Mortals
A good guide to the basics of normalization and database design.
SQL Antipatterns: Avoiding the Pitfalls of Database Programming
It’s as important to know what not to do as it is to know the correct techniques.
SQL Server – Internals:
Pro SQL Server Internals: A very comprehensive look at SQL Server internals and architecture.
SQL Server 2012 Internals & Troubleshooting
The first chapter is the best overview of SQL Server Architecture that I’ve read.
SQL Server – Security:
Securing SQL Server
Denny Cherry covers a wide range of topics, from database encryption options to auditing to granting rights to database objects.
SQL Server – T-SQL:
Anything by Itzik Ben-Gan, but in particular:
Microsoft SQL Server 2012: T-SQL Fundamentals
Seven Databases In Seven Weeks
A good introduction to PostgreSQL, Riak, HBase, MongoDB, CouchDB, Neo4J and Redis.
A good introduction to the varied types of NoSQL database systems.