Used to be in the book trade, and there are (usually) valid reasons for cover changes. These include: film covers, updating the back list so that the older books match the design of the new books, updating what was a strong design a few years back but is now dated or out of style, and a change of style to match a reclassification of the author's work. It only tends to happen when the books are almost all sold, as the publishers will generally have to take back stock with the old covers, so I don't think that they deliberately do it to con people into buying the same book twice! I used to keep a record of the books I had in an organiser, or if you have a PDA there are numerous programs you can download to catalogue your books (many of them are free!).