Generally, the answer to that question is yes. You'll need to to use Adobe Digital Editions, which is the DRM behind most of the ebooks you'll buy online, Amazon and Barnes & Noble notably excepted.

And as Boris said, the eDGe defaults to your enTourage account user name and password. So if you have another Adobe ID out there (maybe you signed up a long time ago for InDesign or PhotoShop and forgot?), then you'll need to reconcile those accounts before you can seamlessly download and open ebooks.

Addendum: You might want to read the Adobe Digital Editions FAQs.