If you choose to download the book to your computer or device (as opposed to reading it online), you will need to install Adobe Digital Editions, which is free of charge.
The Adobe Digital Editions download site is here: http://www.adobe.com/products/digital-editions/download.html
The support site for Adobe Digital Editions is here: http://www.adobe.com/products/digital-editions/faq.html
Sometimes the connection between your Adobe Digital Editions (ADE) accounts can get out of sync. Should you get an error like this:
Unable to Download
Error getting License. License Server Communication Problem:
E_AUTH_BAD_DEVICE_KEY_OR_PKCS12
To re-sync your account:
The following are frequently-asked questions from the Adobe Digital Editions website:
What is Adobe Digital Editions? Adobe Digital Editions software offers an engaging way to view and manage eBooks and other digital publications. Use it to download and purchase digital content, which can be read both online and offline.
Is it a free download? Yes.
Why do I need to activate my computer with my Adobe ID? Some digital publications that you purchase online or check out from a library may contain digital rights management (DRM) protection that restricts the items from being shared with others. Digital Editions helps ensure that your eBooks and other digital publications are not "locked" on one computer. They are assigned to you through an ID, and can be viewed on any compatible device you own that you activate with that ID.
What are the requirements to get an Adobe ID? An Adobe ID is a free and a nonintrusive way of identifying users. You need only provide a name, country, and e-mail address. You can create your Adobe ID when prompted within Adobe Digital Editions, or go directly to the Adobe.com, select “My Adobe” in the top-right corner, then click on “My Adobe ID” and follow the instructions for creating an Adobe ID.
According to a story by Nate Hoffelder, people accessing/downloading ebooks using version 4 of Adobe Digital Editions are potentially susceptible to data breaches, which can track reader use logs. Murphy Library wanted to alert users about this, as it is a privacy concern. Eric Hellman has since updated the this story with additional details (see Correcting Misinformation on the Adobe Privacy Gusher).