Popular books are reprinted several times, and some can even be reprinted more than once in a single year. Authors often take advantage of a reprint to update the information in their books, and to correct mistakes that have been pointed out by readers of earlier editions.

The problem is that many online bookstores do not automatically show you the most up-to-date edition of a book at the top of their search results. So don't assume that just because the author and title match, you've found the book you want to buy. Spend an extra couple of minutes reading through the other entries for the same author, in case there is a more recent copy of the book available. Note that you may have to look at the detailed information on a given book in order to find out exactly when it was printed.

When you shop regularly at online bookstores, you'll find that their pricing can be all over the place from time to time. You may well find that they currently "stock" 3 or more editions of the same book, all at different prices. The cheapest edition may in fact be out-of-date, having been superseded by one of the pricier editions. But that's not always the case, so look closely at the publication date. Some books are published by more than one publisher in quick succession, and the online store may have better terms with one of the publishers and hence be able to discount that edition further.

Don't buy the same book again a few times when shopping online. This can be a surprisingly easy mistake to make, especially with books that have both a "title" and a "subtitle". Many online stores list the book with its full title and subtitle. I ended up buying the book a second time recently from an online store since I couldn't remember owning a book with such a long title - my copy only says "Gates" on it. You can probably understand my mistake when you look at a photo of the cover - now if only the store I used had had photos!

You can also run into problems between the hardback and paperback editions of a given book, since publishers take it upon themselves on occasion to change the title in the transition. So wherever possible, try to look at a picture of the book's cover, and read the brief Synopsis provided by many bookstores. If the book sounds familiar, it may well be sitting on the bookshelf behind you already!