I am really disappointed to know that Sitepoint wants us to buy the same digital books 3 times (one for each format) and I wish you would reconsider this decision. I have bought eighteen of your books in PDF format and other publishers (Manning, O'Reilly and Pragmatic Programmers) allow me to download any version I want when I buy a Digital Edition from them.
After all, I'm buying the content and not the format it is distributed in unless it is printed where I have to pay for the paper as well.
I have an iPhone and a Kindle so I don't believe that I will be willing to pay 3 times for the same book. Considering that I'd have to rebuy 36 books (.mobi and .epub for each of the eighteen books I already own), I probably won't buy any books from you from now.