The Art of Prolog, Second Edition

MIT Press makes this classic text freely available as a PDF here:

https://mitp-content-server.mit.edu/books/content/sectbyfn/b...

(Publishers are making many academic texts freely downloadable in what is called "Open Access", which is what MIT has done here -- there's an Open Access tab on the page linked to by the OP which gets you to the link I provided).