Some thoughts and reminders for those new to the ebook world (and this blog).
Prices on eBooks are quite volatile and can change with no notice, at any moment. The price changes are not on any regular schedule, such as a specific time of day (although around midnight to 1AM in Seattle does see more changes from Amazon), so a free book may be free for only a few hours (or only one) or it may remain free for weeks or months (or even years, in some cases). When the price does change, it may go back to the price it was before, to a new discounted price or to a new all-time high.
Those who live outside the US will often find that the price they pay is different from what might be seen by others. Canada, Australia, the UK and often EU may see the same books as free or they may not -- the book may be full price in all non-US regions or it may have a slight surcharge added (by Amazon or the publisher, depending on who is offering the promotion). The book may not be available at all outside the US (true for almost everything at B&N and some other stores without international publisher contracts). Many books that are free on Kindle end up being $2.00, plus applicable VAT, in a large part of the world. No doubt this represents the higher cost of doing business in those countries, for Amazon, but it is not a "Whispernet delivery fee" -- it's the price of the book if you live in those regions; delivery is still free, whether via WiFi, 3G or USB cable. Keep in mind that in some countries, the contracts for 3G no doubt end up costing Amazon more, plus there are currency conversion issues (which they pay when forwarding money to publishers and taxing authorities there, as well as paying those 3G fees), added administrative and accounting overhead, etc. Considering that most of the other bookstores simply don't allow sales to those regions at all (and how high paper books are in some of the areas), it's still means Kindle books are often quite a bargain, even if you do have to think a bit more before picking up all the "free" books that those in the US can accumulate without thought as to whether they will read them or not.
Always be sure to double check the price and verify that it is still free before clicking, whether at Amazon or another store. The price you see is the price you will pay, regardless of whether it was free earlier or to those in other geographic territories. This is especially necessary if ordering from your Kindle (use "Book Description" from the Menu, not "Buy Now", to see the price before ordering). If you are purchasing on your PC, you'll either see a receipt with the final price (Kobo, B&N, Sony, etc.) or have a link you can click to check the price (Amazon). Be sure to do so. If you buy from your reader, you should get an email with the price you paid that you can check.
If you pre-order, you won't get a receipt right away with some stores (you do with B&N), but you can usually check the status and price you'll pay; with Amazon, you do that on the Manage My Kindle page, which is linked at the top of any Kindle book page. A lot of people get confused on how pre-orders work. First, there is not a lowest-price guarantee on Kindle eBooks, like there is on paper books. This is, no doubt, partly due to Agency Pricing, since Amazon has no control over pricing on those books. If you check Amazon's help on their lowest price guarantee, there are only a few specific categories to which it applies, the items have to be sold by Amazon (no third-party items) and it also requires that you see the Lowest Price Guarantee wording on the product page. So, if the price of a Kindle book goes down during the pre-order period, there is no guarantee that you will pay the lower price. It's true that with some books, Amazon has lowered the price of the pre-order as the book price fell, but with others they do not. It's hit or miss, as to whether any particular non-Agency book will be lowered, which is why I recommend checking the price now and then and if the price on the Manage My Kindle page hasn't come down to match, cancel the pre-order and re-order it.
What Amazon and other stores do guarantee, though, is that you won't be charged more than the price shown when you clicked (free or otherwise). They do require you to have a credit card on file at the time of the pre-order, even on free books, but that is because their program that sets up pre-orders does a check for a valid CC, regardless of price. If the book says $0.00 when you click and that is what is shown on your receipt (and on the Manage My Kindle page), then you will not be charged when the book is delivered. With Amazon, the credit card must be valid at the time of the order, but not necessarily after that. If it is a free pre-order, you can even remove the credit card from the account and the book will be delivered. This won't work at B&N, since your credit card number is used in the DRM scheme; it must be valid when you attempt to download the book, free or not. If you pre-order a non-free book at Amazon, then your current gift card balance will be used at the time of delivery and the credit card charged only if there is an insufficient amount on hand.
The only thing I dislike about gift cards at Amazon, is that you can't set them up to be used only for Kindle books; any Subscribe and Save item will use your gift card balance, as will anything you buy that uses a coupon code, is a lightning deal or personal GoldBox item or is part of a promotional deal. If you try to change your payment on any order with a promotional discount, you'll lose the discount. Other stores don't have that issue, but only because there isn't that much else you can buy from them. That said, using Amazon Gift Certificates or B&N Online Gift Certificates on your account can make it much easier to balance your credit card bill and avoid having your bank freeze the card due to a large number of small amount purchases. Note that with B&N, using gift certificates will not eliminate the need to keep a valid credit or debit card on file pretty much all the time and that every purchase (and download) will see a pre-authorization of a dollar; with credit cards, you never see these pending charges and they expire after a few days, but with debit cards, that amount is frozen in your bank account until the expiration passes.
DRM is an issue that you will run into at every store. Amazon uses a special version of Mobi DRM and the Kindle is not compatible with DRM'd mobi books (it can read DRM-free mobi books and any other non-DRM'd format can be converted using Calibre). Mobipocket and a few others do sell DRM'd mobi books and these can be used on your desktop and with some phone apps, as well as a few older readers. Most other readers now use Adobe's DRM'd EPUB, rather than DRM'd Mobi; it's just as proprietary, but due to being more popular now is considered by some to be "a standard". If you buy books from B&N, they also use a special version of DRM, making their books incompatible with other EPUB readers out there. So, the nook can read B&N and Adobe EPUBs, Sony and Kobo (and others) can read Adobe EPUB and all of them can read non-DRM EPUB and non-DRM'd books in other formats can be converted via Calibre for these readers, as well. Most of the readers support some type of PDF, but the handling and resolution is often poor, if only because PDF is a print format and the readers out today are all smaller than a standard sheet of paper.
Some stores require you to jump thru hoops to get books. Sony requires you to use their desktop software, so you are out of luck if you have a linux box and can't get it to run. Kobo sometimes has books that are only readable on the web and cannot be downloaded. Always check that you see "Adobe DRM EPUB" on the Download options line before purchasing. I've sometimes found the same issue with Borders (which makes sense, since Kobo is providing their ebookstore), but not always on the same books. Getting free books at Borders can always be tricky. I've found that maybe half cannot be added to your library using the Add to Library button on the page - you just get a message that there are Technical Difficulties. For these, open the Borders Desktop software (or find the book on your Border's WiFi Kobo reader, although this is a LOT slower and iffier in whether it will work). Go to the Store, then search on the book title. The free book will almost always be the first one listed in the search results and will say Download underneath, rather than showing a price. Click on Download, let the desktop sync the book to your library. After that, you can transfer it to the Kobo via USB, sync your WiFi Kobo or download the epub from your library via the web.