I've only dabbled in Prolog so far, but enough to be able to thoroughly recommend the book Clause and Effect by Clocksin. As one of the amazon reviewers says 'It serves roughly the same purpose for the Prolog language that "The Little Schemer" serves for Scheme.'

http://www.amazon.com/Clause-Effect-Programming-Working-Prog...

For a preliminary taste, this online tutorial seems quite good:

http://www.coli.uni-saarland.de/~kris/learn-prolog-now/lpnpa...

You might also like to look at this free book:

http://www.ida.liu.se/%7Eulfni/lpp/