I've found Bratko's book to be an amazing tour de force.

http://www.amazon.com/Prolog-Programming-Artificial-Intellig...

Actually, one of the best book for Prolog is book written by my professor Ivan Bratko: http://www.amazon.com/Prolog-Programming-Artificial-Intellig...

We had to write Pascal interpreter in Prolog - that was an experience. I strongly suggest to everybody to try Prolog - then you will understand why it is good idea to use declarative languages as much as possible when building product (including SQL).