I do not know about you, but for me, a book has to be pretty darn exceptional to persuade me to buy a second edition of a book which I already own the first. Unix Power Tools is one such book. It is simply packed with tons of useful tips which the authors have accumulated over decades of using Unix, and is a sort of `crème de la crème' of O'Reilly reference books.
Praise aside, the book is not for everyone. It is an intermediate level reference, not an introductory tutorial. If your problems are like "How do I delete a file?", you should read something else first, get acquainted with Unix, and then return to it. If, however, the questions you face are more like "How do I delete a file with a null name?", then this is exactly the book for you. Unless there is a real Unix wizard around you, this book is likely to earn you this title in your environment.
The second edition focuses on POSIX systems rather than on SysV/BSD, uses Bash and Tcsh instead of Sh, Ksh and Csh, and has moved from Awk to Perl. The two-colour printing is gone, though. Fortunately, the superb index - one of the best I have seen - is still here, and so are the cross-references in the text. Accompanying CD-ROM might be essential if you are living in the mountains of Tajikistan, but anybody connected to the Internet will probably prefer to download newer versions of software on-line.