As usual, there are a ton of formats:
ePub.
- Mac. Nook for the Mac seems like the best reader on you Mac OS X or ePubReader from within Firefox.
- iOS. This is the format that the embedded Books reader understands. You do have to jump through hoops to get it there as it requires a tethered sync to your iPad.
Mobi
- Mac. This is the format that Amazon uses, so you should be able to use the Kindle reader.
- iOS. Same deal, Kindle reader can handle this.
If you like super open source, you can also use Calibre or a long list
