w user...cannot access books

I am using Digital Editions for an online class. I find books that are only available in this format. Once I download them, my Adobe will not offer me an option to purchase. I do not own a ebook reader and the books keep expiring. How can I reauthorize a book once expired, and ...do I have to have an ereader to download and save a book I need for longer than 2 weeks? What reader is best? How can I buy a book before it expires?

Jerry