I just went to the amazon website and figured out the problem you are facing. Don't worry you are not crazy, it does say that the Paper Back version of "Nevermore" is due to come out on the 15th of August.
See from the link - http://www.amazon.co.uk/Maximum-Ride-Nevermore-James-Patterson/dp/009954413X/ref=sr_1_2?s=books&ie=UTF8&qid=1375358868&sr=1-2&keywords=nevermore+james+Patterson
However if you are really desperate to get you hands on this book then you can do what I did and buy the Hardcover version of "Nevermore".
See from the link - http://www.amazon.co.uk/Maximum-Ride-Nevermore-James-Patterson/dp/0099544091/ref=sr_1_1?s=books&ie=UTF8&qid=1375359207&sr=1-1&keywords=nevermore+james+Patterson
The Hardcover version is quite a bit more expensive and is not as comfortable to hold when reading as you may know. If you are looking to get the book a little cheaper then you can always buy a second hand copy in good condition.
Here is a link from where you can compare prices: http://www.amazon.co.uk/gp/offer-listing/1846057779/ref=tmm_hrd_used_olp_sr?ie=UTF8&condition=used
I hope this helped and you get the book in time. Happy reading!
Tell me what you think about the "Angel" book. And the Flash applet I made of the flock flying - if you have not yet seen if go to my MAXIMUM RIDE homepage. (it will not show up if you are viewing from a tablet or iphone/ipad/ipod. You must view it from a laptop/computer.) Hope you like it.