According to Wikipedia there are several variations of the term; e-book, eBook, e-Book, ebook, digital book, or even e-edition. Is there anyone that is most widely accepted or used?
Donald Knuth, inventor of TeX, writes on a related word:
A note on email versus e-mail
Newly coined nonce words of English are often spelled with a hyphen, but the hyphen disappears when the words become widely used. For example, people used to write "non-zero" and "soft-ware" instead of "nonzero" and "software"; the same trend has occurred for hundreds of other words. Thus it's high time for everybody to stop using the archaic spelling "e-mail'". Think of how many keystrokes you will save in your lifetime if you stop now! The form "email" has been well established in England for several years, so I am amazed to see Americans being overly conservative in this regard. (Of course, "email" has been a familiar word in France, Germany, and the Netherlands much longer than in England—but for an entirely different reason.)
Extending the principle, there is no question that "e-book" (and also "Ebook" and "eBook") will go out of style shortly. The only real question is if "ebook" will stick or if we will end up on just "book". It's instructive to note that if someone tells me to mail them a document, I will reach for my computer and not for a paper envelope.
Both are interchangeable, so you can use the hyphen-version as well as the non-hyphen version.
Since this is almost identical to the meta question “To hyphen - or not to hyphen? That is the question!”, I'll simply quote the answer I posted as reply to the meta question to make it complete…
There seems to be no general consensus. Both are apparently valid.
Personally, I would go for the version without the hyphen as that's easier to spell and remember. Trust me, I've got a website named like my nickname and sometimes, that hyphen is killing people. Thinking about it, I could imagine StackExchange came to a similar conclusion when dropping the hyphen.
Yet, that doesn't mean I would like to see edits by spelling-nazis who rant and rave on every hyphen they see. I would simply make both interchangeable, with the hyphen-version being treated as a synonym.
Talking about it, here are some related things I found in the dusty corners of the internet:
Google1 has 499 million results for
eBook, versus only 138 million for
e-Book, 3.8 million for
digital book, and 24.5 million for
e-edition (most of which appear to be newspapers).
So, at least according to Google,
eBook pretty clearly wins.
1 All searches done with exclusion operators (e.g.
"ebook" -"e-book") to stop Google from auto-guessing equivalent words.