Can I download library e-books to read offline?
Depends on the vendor. Most e-books from Ebsco and ebrary can be "borrowed" -- downloaded to be read offline for a certain period, after which they expire, using your Adobe Digital Editions. Almost all e-books can be partially downloaded as pdfs-- a chapter or 60 pages per browser session, and these don't expire. Safari e-books can only be read page-by-page online, unfortunately.
For more information on viewing and downloading e-books, see the E-Books at UMass Amherst Guide.