I personally have not seen such a list, but I would think that if it works on an iPod, it will work on an iPad. One of the best readers out there for non-DRM wrapped media (material that is in the public domain) is called GoodReader.
goodiware.com :: products :: GoodReader
Another great reading app is called Stanza.
Stanza: a Revolution in Reading | Lexcycle With it, you can access several free books sites like Project Gutenberg plus a few others as well as a couple of sites for paid books.
Of course the Apple reader is called iBooks and it is free. You buy books from within the app and those books must be read on iBooks. The iBooks app is also a very good PDF reader.
The oldest and largest content provider is
Amazon. You buy books at
amazon.com, but you read them only on the Kindle app which is also free.
Both Borders and Barnes & Noble have readers, but they lag far behind
Amazon in terms of content.
Does this help or are you more confused?