First thing I want to mention is that this book is NOT from 1977. Per Niklaus Wirth:
"The language Oberon, published in 1988, has a long history. It is a descendant of Algol 60 (1960), Pascal (1970), and Modula (1979)"
Per Wirth, the first edition of this book dates to 1996 and the book was slightly revised in 2005. I am currently reading the latter version. The book uses Oberon-0 a predecessor to Oberon.
Now as to the coverage of subject matter: This book is short, but it offered me the most precise, pragmatic and readable definitions of everything around compiler technology I have read so far. I am reading both versions of Aho (Lam), Sethi, Ullman 1989 and 2006 and although more thorough and very formal, they seem to me less precise than the formalisms and explanations that Wirth provides.
This book is written by someone that's been there, done that. A person that has lived through all the complexity then thrown it away, demystified and distilled it to its essential components.