At Amazon's main site, there are listings here for the original trade paperback, original large print edition, and the revised Kindle edition, and here for the smaller format, bargain trade paperback, and here for the revised regular trade paperback. Both paperback and Kindle editions are also available at Amazon's international sites.
At Barnes and Noble, the trade paperbacks can be found here, the large print edition here, and the Nook book here.
Or buy the ebook at Kobo.
Or at Smashwords (various formats).
Or, for links to various retailers (including some of the above), go to the listing at Goodreads, and click on the 'stores' button in the Get A Copy area. That would include a link for the ebook at the Apple store.
If I've missed a listing for one of your favorite retailer's, or to your own bookstore (assuming you carry this title), please feel free to share a link in the comments.
Also, if you like to read ebooks through your local library, please ask them to add it to their catalog if they haven't already. For now at least, they should be able to get the Smashwords edition for free, via a special deal offered to libraries.
There were very few changes between the 2011 edition and the 2014 editions. The spelling of a place name got corrected, and there were a few minor changes made, mostly with an eye (or should I say ear?) to making it so it would read better out loud if ever it got put into an audio book.