Mark Crump, in a review at GigaOm of iBooks 3, said this:
Unfortunately, one area Amazon still eats Apple’s lunch is how it handles sideloaded content. You can upload damn near anything to your Personal Documents section on Amazon and download it to your various Kindle apps. Apple only supports this feature for books you buy through the iBookstore. If you sideload a lot of content to iBooks (guilty) you still need to use iTunes to sync it to your iOS devices.
Nope. I haven’t used iTunes to sideload books into iBooks for years. It’s completely unnecessary. Instead, just email the book to yourself, then tap on it in the iPad’s Mail app, & it will ask if you want to open the book in iBooks (which actually copies it to iBooks, & then opens it). Boom. Done.
If they book is too big for your email program, then put in Dropbox, & select the book in your Dropbox app on your iPad. From there, you can open the book in iBooks (which again, copies & opens).
It really couldn’t be easier.