With what progrm are you trying to read the book?
The Adobe ADEPT DRM system is a badly implemented pain;
bad enough for experienced users when it works properly, and appalling for 'typical' users or in the frequent cases where it doesn't work smoothly.
Neverthless, once you eventually hack your way through the bushes, it usually works reasonably easily.
You can change the email associated with your AdobeID.
Logon to your AdobeID at any Adobe site (eg https://www.adobe.com/account/sign-in.adobedotcom.html?returnURL=https%3A%2F%2Fwww.adobe.c om%2F ) using your old email.
At the top right under you r old email name, open up 'My Information'
On that page you can change the email address associated with your AdobeID to your new email address.
You will need to 'save my changes' and verify the email they will send to your new email address.
That said, once you have authorized your copy of Adobe Digital Editions (ADE) and your Kobo,
you shouldn't need to use/remember the email address associated with the ID anyway for day-to-day borrowing, copying to ther Kobo, and reading.
I've been reading books reasonably happily from the Hampshire library on Cybook and Sony ereaders.
There is one other issue you may face though. There are (lots of) bugs in ADE2.0:
one of these bugs is that it does not work with Overdrive library books (such as Hampshire uses) for some eReaders, including the Sony.
I have also heard of similar issues with Kobo.
You may need to install the older but more reliable v1.7.2.
Version 1.7.2, it is a little difficult to find, available on Adobe site for Windows and for Mac.
The forum software is sometimes corrupting the link above. There shouldn't be a blank in 'editio ns.html'. The following redirects to the same page: http://tinyurl.com/diged172