Three eBook Apps lead the field, and at these prices -- $0.00 in each case -- you'll probably want to have all three apps on your iPad, iPhone, or iPod Touch. For now, the Kindle Store has by far the best selection and prices that are equal to or lower than its competitors, but with all three apps on board, it is easy to compare prices.
Just click on any of these links directly from your iPad, iPhone, or iPod Touch to download the app directly to your device.
Kindle - $0.00 - Over 650,000 Titles
iBooks - $0.00 - Over 60,000 Titles
Barnes and Noble eReader - $0.00 - Over 300,000 Titles