Copy .epub files to iPAD seems to be very easy.

1 Download the .epub file to your computer.

2 Open up iTunes and select File -> Add to Library… menu item. From the file chooser, select the .epub file that you want to transfer to your iPad. Click Choose and the eBook file will be added to your Book Library in iTunes 9.1.

3 Sync as usual and the book will show up in iBooks.

ePub

EPUB is a free and open e-book standard by the International Digital Publishing Forum (IDPF). Files have the extension .epub.

EPUB is designed for reflowable content, meaning that the text display can be optimized for the particular display device used by the reader of the EPUB-formatted book. The format is meant to function as a single format that publishers and conversion houses can use in-house, as well as for distribution and sale.