I cannot open a book I purchased and opened earlier. It said it is tied to another account

Once you download a DRM Ebook for use on one account you cannot read it on a device registered with a different account.

You can have up to 6 devices registered to an account; if you are within that limit you can deregister a device and reregister to the account associated with the book.

However, you will then NOT be able to read the DRM ebooks on the device associated with its original id.

Once you get two accounts somehow, you have to balance up the least inconvient way to you of using the two accounts.

~~~

Another possibility is that ADE has got its registration screwed up; and says it is registered with your id but is only parly registered.

In that case, remove the registration (ctrl-shift-D to ADE for a computer, ctrl-shift-E to ADE with the device plugged in for a device, or cmd-shift-D/E respectively if ADE is running on a Mac)

restart ADE, and reregister the ID.

Moved to http://forums.adobe.com/community/adobe_digital_editions

Did you open it with Reader? Are you trying to open it with Reader now?