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Notes on Book Availability:
There is no way to really say what is or will be soon available on each of these readers. Generalizations could get me in a lot of trouble, so I'm going to make it simple. I will explain how you publish to each of these readers and then you can use your brain to decide which one will have a better selection of available books.
Kindle: Kindle publishing requires a file of the eBook and a cover image. You are assigned an ASIN number (Kindle ID number) and do NOT need an ISBN. Anyone can publish to the Kindle with an Amazon account, meaning that there are indie books as well as professional books available. There is a small fee for some document conversions to be sent over 3G, but you can do conversion without Whispernet (the 3G) for free. If you have the 2nd generation, you'll need to plug in to a USB cable, but 3rd gen users can use Wi-Fi. You can load your own documents by looking up your account email and tagging free. in front of the kindle.com (example yourname@free.kindle.com). The Kindle does not display color.
Nook: Barnes & Noble lets you publish to the Nook without an ISBN through their PubIt! site. Also, B&N users can run a select number of apps, PDFs, and can read eBooks for free within the store, as if they were doing so with a real book.
iPad: iBook publishing requires purchasing an ISBN for the eBook edition, which can cost $125 dollars a book if you are an indie (big publishers buy in bulk and can get ISBNs for a buck a pop). HOWEVER, the iPad runs Kindle and Kobo apps, so you can read indie books on the free Kindle app.
Kobo: The Kobo requires an ISBN like the iBook publishing. You cannot simply submit to the store, either. You have to inquire for a publisher kit and fill out a form by email. They take a few days to get back to you, to. Kobo is not indie friendly.
Sony: While you can use Smashwords to publish to the Sony Reader, companies who don't want to use the indie service have to go through an email process to submit a release, much like with the Kobo. Sony books can also be borrowed from a library. The real problem plaguing Sony eBook publishing is that in an attempt at Digital Rights management, to let Libraries loan Harper Collins eBooks, Sony has made them self-destruct after 26 loans. The implication here is that real books are unreadable after being checked out 26 times (which we know isn't usually true) and that rather than embrace the improvement on the traditional book, Sony has gotten greedy and wants to make PUBLIC LIBRARIES keep paying for books. This is turning libraries off and making the library availability feature a moot point.