1. Prolog is often used in things that are kept very secret. There is a relatively small set of professional and highly committed users centered on distributions like SICStus. There is a much larger set of hobbyist/academic users centered on distributions like SWI, Tau, Ciao! and so on. The feature sets of the commercial distributions gives you some idea about the sort of capabilities the hardcore users require.
2. Getting good at solving problems with Prolog is a borderline mystical experience -- or at least it is for me. Seeing the logical structure of problems, their representation through symbolic pattern matching and their solutions as search, is an insight which once seen cannot be unseen. It will condition your perception of problems forever more.