For those who need some context: In 1983 David Warren published a paper describing an abstract machine that could be used as the target of a Prolog compiler. This machine became the basis of most Prolog compilers - it is much faster than interpreters. His paper was not easy to understand. Hasan Air-Kaci's book was a brilliant exposition of Warren's work, and was a must-read for anyone serious about working on Prolog interpreters or compilers.

Oh dear, that reminds me of one of my courses I had to take where we had to memorise the WAM and execute it on paper in the exam. Most useless course ever.