For library books, install the Overdrive app on the iPad; that can borrow, download and read Overdrive library books (as used in many countries).
For other DRM .epub books, install the Bluefire app on the iPad.
Install ADE on a PC/Mac, and make sure your ADE and Bluefire are registered with the same AdobeID.
Download .epub books to the PC using ADE and then copy them to the iPad and read with Bluefire.
Unfortunately, Apple likes to make what should be a simple copy operation difficult; so you have to copy indirectly, for example using iTunes, Dropbox or by emailing to yourself.