This page is for sharing literature and tools to help everybody in their studies. Examples include wordlists, books freely available on the Web, LaTeX style files, software you have written... go ahead!
Covington: Prolog Programming in Depth
Prolog Programming in Depth can serve you as a very nice reference to Prolog, as it is a searchable PDF and also very straight forward and systematically arranged. As an introduction to Prolog it is also more accessible than The Art of Prolog. You can download it here.