Back in the last millennium, someone produced an "electronic book." The term quickly got condensed, like "electronic mail" did, but the condensation never became standardized.
I thought that "eBook" looked cute, structured like "iPad" and "iPhone" (but they are trademarks and should look cute). I wrote "eBook" and published it in several books.
At some point, I decided to change to "e-book." I even went so far as to change 211 occurrences in a book before it was published.
Then I realized that eventually the hyphen would probably disappear, as "e-mail" has transitioned to "email."
And I realized that at the beginning of a sentence, "e-book" becomes "E-book," which may be construed as yet another variation, not merely the beginning of a sentence.
New words often start out as hyphenated terms, and then close-up and lose the hyphen. "Over-time" became "overtime." "Infra-red" became "infrared." Maybe "microwave" was "micro-wave."
So, it looks like the world is heading for "ebook." I'm just not ready yet. But I do write "email." Language is consistently inconsistent. Why do we have compact discs and floppy disks?