Authorizing more than one device, and downloading ebooks (for use without Internet).

I'm confused about how to use ADE. I want to be able to download an ebook, say from Gutenberg, or checkout or buy a DRM book, and--

1. Have it available on more than one device.

2. Have it available on more than one device without an Interent connection.

I authorized one device, but when I tried to authorize a 2nd device, I got an error message that one device had already been authorized with the Adobe ID. 

Confused!