Modula-3 is one of the great milestones in programming language design.
It is not, as one of the reviewers of this book has said,
a language whose deficiencies were corrected by C++.
On the contrary, Modula-3 was developed after C++ (in the late 80s) and
in fact corrects many of the deficiencies of that language. Many
of the features of Java and C# were lifted more or less intactly
from Modula-3, and the designers of Java have regretted that
they didn't lift more, as they have had to add generics later.
That being said, this is a terrible book. It is poorly organized
and poorly written.
Greg Nelson's "Systems Programming with Modula-3" is the best book, and most of it is available for
free on the Web. Harbison's "Modula-3" is a good introduction
for someone who just wants to know how to write basic programs
in the language. Unfortunately neither of these books is in
print any longer...