Have you seen: Useful Prolog references
There are a few excellent and free books listed. You really should learn from a book or site such as Learn Prolog Now! instead of making up your own problems to learn. But once you know the basics, I always suggest that real world problems will help you learn more than a book.
While the syntax is slightly different you might find this site of use.
I note it because:
- It might be of help to you. I know when I was learning unification it would have helped me.
- I am curious to know if someone learning Prolog on their own finds the site of use and what problems they have with the site. The site has a few warts and with some enhancements/changes it could be very effective for new users.