Amazon is undergoing a brand change in Canada and the United States and currently A/B testing a new e-book naming system. Right now thousands of users are reporting that Kindle eBook and Kindle Edition i s being changed to Digital Book or eBook.
I think Amazon is changing all references to Kindle for a couple of reasons. When the company first started releasing tablets they called them Kindle Fire, then eventually phased out the word Kindle and now just refer to them as Fire Tablets. You can also have e-books read aloud to you with the Amazon Echo and Alexa in general.
Removing the word Kindle from e-books informs users that they do not need an e-reader to read e-books, but there are now many different mediums Amazon employs for people to read or have books read aloud.