I've only been focusing on learning more about security since the beginning of the year and I have a bit of a different view on books.
I own a few different security books, grey hat hacking, hacking exposed and they are pretty much similar in content (which you would expect), but it depends on your skill level and the way you learn.
In all honestly I only use books as a reference point rather than reading through the whole thing, and forgetting most of it. The book depends on what you want/need to learn, for the myself I wrote a training plan over a year covering different tools and methods and then find the books (and don't forget Google) to learn those particular areas. I throw in a healthy dose of lab work (built various VM's) to push home what I've read and learnt.
Pen Testing usually follows a set pattern in terms of what you need to do:
scan, enumerated, exploit etc etc
From that you can work out the areas you need to learn about and then find books/pdf's/google material to progress. Otherwise you might end up reading books about subjects you don't need or only just cover a subject that is really important.
But then that's the just the way I learn best, like I said everyone is different and there are a lot of good books available.
I've just created a new wish list of Security books. Check it out here.http://www.amazon.co.uk/registry/wishlist/1INPZOXT8TJY3