There are about a thousand different paper editions of Edgar Rice Burroughs’ John Carter series and there are almost as many ebook editions.  Probably the best was put together with loving care by ebook lover HarryT at Mobile Read Forums.  He has painstaikingly crafted a single volume with all eleven Barsoom novels together.  You don’t have to be a Mobile Read member to get it, but Mobile Read is definitely worth your time.  Here is a link to the ebook:

http://www.mobileread.com/forums/attachment.php?attachmentid=34895&d=1251219101