The Power of Prolog
Prolog is the only language that I've ever learned that feels like magic. Of course, it's not, but it feels like that. I'm still learning it, but I feel it and the logic paradigm are underused. I wonder if it's fair to consider it constraint-based programming rather than just logic programming. I used the book Thinking as Computation to learn it (and still am), and that's what it felt like we were doing in the book: just setting up constraints for the program to run through and find matches for the constraints.