Today I found this free PDF copy of the book Logic, Programming and Prolog (2nd edition), made available by the authors. The book discusses Prolog with a formal logic perspective and it wastes no time jumping into the logic. It is probably not the best starting point for those interested in Prolog but lacking a formal logic background. I am planning to read this book because although I have a formal logic background, a lot of my Prolog education was not with a formal logic perspective. I'm hoping this book will help connect my education in logic with the knowledge of Prolog to help me write better code.
For those looking to get a lighter introduction to Prolog, there is always the free online version of the excellent Learn Prolog Now!. I first learned Prolog from this website five or six years ago and I highly recommend it.