Originally Posted by calvin-c
What you mean, I think, is that you have books from many sources or maybe in many formats. If they're all on your iPad then that would be your platform. If you import them into Calibre (not sure how to do that-I avoid i appliances) it will show you the format of the book and will allow you to filter on format. The format determines which app you can use to read them-unless DRM gets in the way. Calibre usually handles DRM pretty well though.
That's more or less what I have. I get books from various sources (Amazon, Baen, Sony, PD, iTunes and a few others) that I read on my iPad. Most sources will allow you to download your books to your computer. I generally download them onto my computer and put them in calibre. Calibre shows the format of the book. From my personal point of view, I like having a copy of all my books and I like to remove the DRM of those books so if the original source goes belly up a la fictionwise and a number of others over the years, I don't lose access to the book I purchased.