Locked out of some of my ibooks because some books I purchased several years ago had a different account username and password.

Changes made to user names and passwords for Apple services, i.e., purchased iBook titles, should be systematic changes made to all of a users account purchases to preserve their ownership. I tried to reread a favorite iBook "I own(ed)" recently, but was denied access because I could not remember a password for an old user account. If I would have tried to change the password for the given username, it would have been sent to a distant galaxy in space. So I changed the user name... and guess what, I got the same error message. A "BIG" corporation, like APPLE, should have "BIG" fore-thoughts and prepare for such events. So how do I get to read a favorite book I purchased under my maiden name now that I'm married to a new identity?