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.
+1 on all of those books. If you are interested in multi-dimensional indices (R-Trees, M-Trees, and many more exotic ones). You should checkout "Foundations of Multidimensional and Metric Data Structures" by Hanan Samet. Very comprehensive! "Database Systems: The Complete Book" is fantastic (pick up the previous version it is cheaper!) but it only touches on multidimensional indexing.
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.