3rd Edition, by Apt, de Boer, Olderog Entire book available online through UIUC library (go to link above and click on SpringerLink - Full Text Online Service).
Assigning Meanings to Programs , Robert W. Floyd In Schwartz, J.T. Mathematical Aspects of Computer Science. Proceedings of Symposium on Applied Mathematics. 19. American Mathematical Society. pp. 19--32, 1967.