Although this text is always mentioned in the same breath as other introductory textbooks on prolog, I don't think I've ever seen it described as "the best."
The book which usually takes the palm in such comparisons is "Art of Prolog." While "Art of Prolog" is an outstanding book, I think that now, in 2006, it has been eclipsed by the 3rd edition Bratko's book. Why?

Simply this: Bratko's textbook is (as far as I'm aware) the _only_ textbook on prolog which treats the language as a living, developing language! Other textbooks are great for their time, but they are unfortunately stuck in their time. Its as if nothing has happend to the prolog language since February 16, 1987. But this isn't true at all!

The biggest case in point: constraint logic programming! Bratko's text is the only introductory prolog textbook to even acknowledge the existance of CLP. And Bratko gives very lucid descriptions of it, along with very helpful examples and challenging exercises.

Another case in point: inductive logic programming! An entire new branch of machine learning theory has risen, based on logic programming, and NONE of the other introductory prolog textbooks cover it? Come on guys!

I would love to see a 4th edition of this book, because since this one has been published, logic programming has moved even further ahead. Constraint handling rules (CHR), logical functional languages (like Curry), using prolog for the semantic web, etc etc etc. It might be the best kept secret in computer science, but logic programming is really still one of the most exciting areas of programming, and Bratko's book does the best job of staying abreast of, and conveying the excitment of, this living and dynamic field.