ADE lets you obtain ebooks and transfer them to the Mac.. However, ADE
won't link up with the iPad, iPod or iPhone and allow you to transfer
ebooks to them. You'll have to do that with Mac software.
As Frustrated said, you will need to transfer the books using ITunes, but you won't be able to read the books in iBooks as it doesn't support Adobe DRM. While there are a fair amount of choices, I would suggest using Bluefire Reader on you iPad for reading.
Bluefire has a tutorial for transferring Library books here: http://www.bluefirereader.com/using-library-books.html
Except for the steps specific for getting a library account, this will also work for all of your other content.