There is a Prolog course (at Birmingham University) available over the web at
http://www.cs.bham.ac.uk/~pjh/prolog_course/se207.html
This site also has information about other public domain Prologs.

Amzi!’s site has provides several Prolog learning aids:
http://www.amzi.com/products/learning_products.htm

There used to be another Prolog course called 'Adventures in Prolog' at:
http://meru.cs.uwyo.edu/~karl/classes/prolog-tutorial/advtop.htm

Finally, there used to be a guide to Prolog programming at:
http://kti.ms.mff.cuni.cz/~bartak/prolog/index.html