Penetration Testing is a field that interest many people, being able to attack a network legally and find vulnerabilities in a network is fun to a select few.
Gaining the knowledge needed to be able to do a Penetration Audit is always a ongoing challenge, with reading and implementing new tactics and ideas.
There is a lot of information to be found online and one could argue all you ever need is Google and time to read. But as much information that is out there sometimes it can be scrambled and not give the whole picture of what is going on.
That is where a good book or multiple books come in to help and put the entire picture in our head to better understand.
Many times a author of one book will explain something very well but lack in explaining something else. For example Nmap which has entire books devoted to it is often talked about but only in passing much of the time. This is a mistake in my opinion and a good understanding of Nmap will give a pen tester a huge edge up when it comes to scanning networks.