A little theory goes a long way, at times. While I'm not by any means a full-time database practitioner, I've always had what some would call a dilletante's interest in DB theory and practice.
One of the most frequently visited Useful Links pages on my web site is the one that assembles a dense collection of database theory and practice topics:
Sharing Knowledge of Highly Efficient Database Design dedicated to database. There are free and inexpensive database eBooks there. (I've never met Alf and I don't have any commercial relationship with him.)
And he's just added a new article that's really worth reading:
Database theory and practice: Advice and investigations