For an overview of basic concepts in DB implementation, I quite like Database Systems: The Complete Book (By Widom I think...). You could do a lot worse than An Introduction to Database Systems (CJ Date), although many dislike his opinionated style :-).

If you're talking about actually implementing a full transactional database system, strong foundational books are:

Transaction Processing: Concepts and Techniques (Gray and Reuter)

Transactional Information Systems: Theory, Algorithms, and the Practice of Concurrency Control and Recovery (Vossen and Weikum)

Neither are exactly easy reading, but the concepts therein are really important.