I've received a few questions already about the best way to read "Windows 8.1 Book" on Kindle.
The best Kindle experience at the moment is with an e-ink-based Kindle e-book reader like the Kindle Paperwhite. (This will change when we release the native Kindle version of "Windows 8.1 Book") For these devices, you should download the Mobi version of the book. That's because Mobi (and ePub) offer dynamic layouts and font sizes, whereas the PDF is a fixed format, where every page is like an image file.
Some have noticed that they can't send this file wirelessly to their Kindle because it already exceeds the 50 MB file size limit imposed by Amazon. This will of course only get worse as the book grows. So the solution is to transfer the book to your device from your PC using USB, where there is no such limit.
To do so, connect your Kindle e-book reader to the PC via USB. If your device is protected by a passcode, you will need to enter that passcode.
In Windows, open File Explorer and navigate to your Kindle.
Then, just copy the Mobi version of the book file into the documents folder.
Now, disconnect the Kindle from the PC. When you navigate to the Home screen on the device, you'll see that the book is now available and marked as "New."
When you read the book with the device, it should look and work like most Kindle books. You can change the font size, for example, the font, line spacing, and margins normally.
To remove old copies of the book, tap and hold on the book title from the Home screen and select "Delete This Book" from the menu that appears.
The Kindle apps on other platforms are a different matter. My understanding is that you cannot copy Mobi files to these devices, but if anyone knows anything about this, let me know. Also, I don't have a Nook device, yet, but I'll be getting one to test the book on it. My understanding there is that ePub is the way to go and that it should work much like the instructions noted above. But more on this soon.
I'm also looking into e-book readers for Windows 8.x/RT. One possible solution is Vibe Reader, which looks OK (and works with ePub), but isn't yet optimized for Windows 8.1 (i.e. doesn't respect the new Snap functionality).
But if anyone has an app they use/like, please let me know.