The key material covered in this book is : - the Unix file system, (Bourne) shell commands & script programming, text processing (using grep, awk and sed), file I/O, system calls and document processing (using troff/nroff, etc).
The book is written in a traditional, no-frills format but is easy to follow, with clear code examples.
Systems programming is not covered any great depth (To fill this gap I would recommend the advanced Unix programming books by Rochkind and Stevens).
For a book published 15 years ago, it inevitably misses newer additions (no coverage of C-shell, K-shell, vi, Perl, etc.). Despite this it is still the classic introduction to Unix programming.