They explain it like this:<p><i>An earlier book by one of us, [Peyton Jones 1987], covers similar material to this one, but in a less practically oriented style. Our intention is that a student should be able to follow a course on functional-language implementations using the present book alone, without reference to the other.
The scope of this book is somewhat more modest, corresponding to Parts 2 and 3 of [Peyton Jones 1987]. Part 1 of the latter, which discusses how a high-level functional language can be translated into a core language, is not covered here at all.</i>