You probably got the first (1986) edition. The second (1994) edition is much better, but more expensive. (But, there's a $49 copy on amazon. That's a steal! I p

You probably got the first (1986) edition. The second (1994) edition is much better, but more expensive. (But, there's a $49 copy on amazon. That's a steal! I paid $80ish, argh...) They added quite a bit of material to the latter chapters, and it updated to the newer, then de-facto standard Prolog (which became an ISO standard in 1995). Useful Prolog implementations usually have non-ISO extensions for module systems, constraint programming, etc. I have both. I keep the first edition at the office, and it's my lending copy.