I have books in the Kindle app, iBooks, Bluefire reader app and Kobo. I need so many apps because some books are only readable through certain apps.
I am looking for a way to tie the books together on the iPad. I don't need to read them in one spot, just have a list of books and where they are located so I dont have to search each app to see where a book is.
Unfortunately, there is not an app that can go to each ebook app and give you a listing of what book you have in each app. BUT if enough people ask, and a developer can be found to contact each app owner like you've listed and give that third party developer the ability to pull from each app, you can bet there will be an app to do that librarian task.
Given the competitive nature of Apple and Amazon and Barnes and Noble, I would be surprised if someone can break down those barriers. There is an app called Leatherbound that will look up books offered by Apple/Amazon/Nook without you doing that search on each website looking for a new book.
This isn't an app, but is freeware that's popular for e-book management:
calibre - E-book management