I'm only eight chapters into it, but The Art of Prolog by Sterling and Shapiro has been quite good so far. It started with several chapters about logical & declarative programming, and is now getting into the Prolog language's implementation of it.
Also, there's http://learnprolognow.org/ , and there's also some interesting material about Prolog and unification in PAIP.
I'm not sure how directly useful Prolog is (and I'm expecting naysayers to come marching in any minute now), but Prolog/Horn clauses seem really handy as a notation. After working through some examples in the book, I have a much clearer idea of how the unification for H-M type inference is implemented. (Much like how thinking about text patterns became easier once I learned regular expressions.)
Also, there's http://learnprolognow.org/ , and there's also some interesting material about Prolog and unification in PAIP.
I'm not sure how directly useful Prolog is (and I'm expecting naysayers to come marching in any minute now), but Prolog/Horn clauses seem really handy as a notation. After working through some examples in the book, I have a much clearer idea of how the unification for H-M type inference is implemented. (Much like how thinking about text patterns became easier once I learned regular expressions.)