How do I download an ebook so I can read it on several computers
It sounds like you need to create an Adobe ID. An Adobe ID is a user account that gives you access to protected eBooks from an OverDrive collection (for example, titles in the EPUB format). Adobe allows you to activate up to six devices using the same Adobe ID.
You can create one on Adobe's website. A link is also provided when you install Adobe Digital Editions on a computer or download an eBook for the first time via OverDrive's mobile app.
Example 1: Prompt to authorize with an Adobe ID from a computer
Let me know if you still need help:)