FYI: I use 3 ebook readers on my iPhone, and sometimes that is not enough. Setting bookmarks in an ebook, and if the ebook reader has a way to create folder, tags, categories, whatever, I can put an in progress ebook in their as a way to easily find it again (and sometimes that even works).
The problem is that the different ebook readers have different sources for the ebooks, so it is not like I can juggle any ebook between different ebook readers.
What I would like is an iPhone ability to create a something similar to an iPhone web-app that has the book cover as the icon, and when I click on it, it opens the ebooks reader that created it, and goes to the last read page. I could collect those ebook covers in an iPhone app folder, and easily continue reading any ebook I was currently multiplexing between others. And when I finished the ebook, since the book cover was just a URL into the ebook app, I could just delete it from my iPhone. The actually book would still be inside the ebook reader, so I could re-read the book anytime I chose in the future.
One can dream.