This is a modern applied approach to using OpenSSH.
Compared with the older "SSH: The Secure Shell" from Barrett & Silverman this book is far more applied and less theoretical. It also focuses on OpenSSH running on Linux/Unix. He assumes Windows clients will be running PuTTY, FileZilla/WinSCP or OpenSSH via Cygwin. SSH Communications latest tools are also covered, but the book's focus is OpenSSH.
Stahnke's approach is to explain how OpenSSH is built and configured and make suggestions he thinks are sensible as he goes along. He makes it quite clear when he thinks rlogin/rsh is safe and when it's not and you should be using SSH.
I've not read the second edition of SSH: The Secure Shell, so I can't comment on what is in this book, and how it compares with Stahnke.
If you know OpenSSH inside out then you'll probably not benefit from this book, but if you want to know or need to know about OpenSSH then this is a good book to get you going.
If you want practical applied advice, then this is a useful book.