Is this reading with Adobe Digital Editions (ADE) or on a eReader.
If ADE, which version?
Has your version of ADE got is registration confused somehow?
On ADE2.0, menu~Help~Authorization Information (or ctrl-shift-I) will tell you who it is registered to (and also give the guid for the adobeID)
On ADE1.7.2, menu~Library~Authorize Computer.
Sometimes ADE gets registration in an inconsistent state.
Not sure if it ever manages to report an ID when the ID isn't really working, but just in case ...
That can be sorted by deregistering (ctrl-shift-D to ADE (cmd-shift-D on Mac)), restart ADE, and register again.
If you want to get technical, you can cross-check the ID.
ADE2.0 will give the guid for your AdobeID (as above).
If you open the .epub book as a zip archive (eg with 7zip, or even rename it as a zip file) and look in META-INF\rights.xml,
it will give the guid for the userid to which the book is registered (in the <user> tag).
Not quite sure what you mean by "which version of ADE", This is a 'Screen shot' of what I was/am looking at.. my original Adobe user ID is still the same as when I installed the book last yr, and I haven't changed computers or drives. Since my question I posted yesterday, I've also contacted the e-book provider (eBookMall) that I had originally purchased it from, and they are looking into the matter as well. So here's the 'screen shot' for your perusal.
That looks like Digital Editions (ADE) v1.7.2 or thereabouts.
That rules out lots of potential problems: Adobe have since updated to v2.0 and introduced lots of bugs in the process.
I can't make out what the dialog box says ~ probably doesn't matter.
It might still be worth going through the deauthorize (ctrl-shift-D) and reauthorize as I described above.
Took awhile, but think I know the root of the problem. When I went in to open the book yesterday, Adobe asked (recommended) that I register my computer to my e-address (that thru me off as Adobe hadn't asked that last yr), and I gave the one I always use now (I have two), and I didn't realize I had used my other e-address initially back in 2012 as my Adobe User Account ID. So now it seems I have two Adobe ID's, the problem is how do I 'change' the 'authorized user' e-address on my computer to the old e-mail address, i.e. so it will allow me to open the book. Here's the two other snipped screen shots.. btw, if you click on the prior screen shot or this one, it will expand so it's more readable.
You need first to remove the authorization you now have with your new email, enter ctrl-shift-D (cmd-shift-D on Mac) to ADE when in Library view.
Restart ADE, and you should then be able to authorize with your original email/ID. (It may just ask, or menu/Library/Authorize Computer)