Clean code, clean layout, with helpful mini indexes. Tech report prose. Recommended good reading even for those who never have to work with compilers. Good to finally have it in English. Prolog is a language filled with interesting implementation problems, and this book is an enlightening collection of solutions offered by various implementations.
English translation is stiff in places, but never mind.
Recommended reading for serious language types. The CWEB System of Structured Documentation Addison-Wesley, 1994. The Program . It makes a good benchmark, a pointer to one possible destination. W. F. Clocksin and C. S. Mellish Programming in Prolog Springer-Verlag, fourth edition, 1994.