Comment by linguae

I used The Art of Prolog, Second Edition by Leon Sterling and Ehud Shapiro (https://mitpress.mit.edu/books/art-prolog-second-edition), which is available for free using the "Open Access" link, and also Smalltalk-80: The Language and Its Implementation by Adele Goldberg and David Robson (http://stephane.ducasse.free.fr/FreeBooks/BlueBook/Bluebook....), which is also available legally for free with permission (see http://stephane.ducasse.free.fr/FreeBooks.html for other freely-available Smalltalk books that are out of print).