I started learning how to program in Prolog a few years ago. I found Prolog to be very difficult to understand, mostly because I did not understand what symbolic logic was. I then purchased a significant number of books on symbolic logic and started studying them. I don’t think it is possible to understand Prolog without first understanding how symbolic logic works.

My recommendation for someone who is interested in learning how to program in Prolog is to start by reading “Logic for Problem Solving” by Robert Kowalski:

http://www.amazon.com/Problem-Solving-Revisited-Robert-Kowal...

I am currently about halfway through an earlier edition of this book, and it is the first book I have found that clearly explains the ideas upon which Prolog is based.