Do you need and ebook version of your just published paperback? 
I would seriously consider it. 
Is it difficult? Not for people who know how to do it.

It's like vinyl flooring. I know how to build cabinets, lay brick and block, finish concrete and, basically, build a house from the ground up. When it comes to installing vinyl flooring, I hire an expert. I've done it a time or two, but it was very difficult and wasn't worth the extra time and effort it took to figure out how to do it.
When it comes to ebooks, I will hire a formatting expert.
Two of the more popular ebook converters are Smashwords and Bookbaby.  Bookbaby charges $249 one time (usually on sale at $199), takes no commissions from any books you sell online, and formats your manuscript for Kindel, Nook, Ibook, Sony and several other online ebook sellers. Smashwords charges nothing for the above services, but takes a %15 commission for each book sold.
You will have to sell a lot of ebooks (around 100) just to break even and justify paying $249 up-front with Bookbaby. I converted my book on faith with Bookbaby, but then it was only $99. I am hesitant to pay their current initial charge. A good comparison of Smashwords and Bookbaby is offered by the To Publish or Not to Publish blog. I used Bookbaby for my first ebook and am thinking about trying Smashwords with my second. Be aware, Smashwords has no phone contact number; they only communicate through email. But Smashwords has been able to negotiate better with Amazon and other online sellers, so your commission is a bit higher than with Bookbaby. Either one is good in the long run.
An ebook requires its own ISBN—You supply the ISBN (yes, you still want to use your own ISBN).

It will be a different ISBN than the one for your paperback.

Ebooks are relatively new and questions are still unanswered about ebook ISBNs.
Read about a recent survey originating in the UK, "Ebook ISBN Mess Still Needs Sorting Out, say UK publishers."

The question is: Do I need one ISBN for an ebook,
 or, will I need a separate ISBN for each e-book format?

According to the UK survey, "Booksellers and wholesalers favor a different ISBN for each format, while most publishers prefer a single identifier."
Andy Weissberg, of R.R. Bowker, touched on the e-book/ISBN question in a 2010 interview. This is a lengthy article but has accurate information about the future of ISBNs. The e-book question is addressed about nine paragraphs in (look for the 2nd light-brown paragraph).
In the article, R.R. Bowker agrees with the International ISBN Agency when he says, "e-book formats should be assigned separate ISBNs, especially where trading models involve multiple partners."
*Ron Pramschufer leans toward the eventual probability that e-books might need multiple ISBNs, one for each of three formats but thinks one will do for now.
What about children's books for the ebook market? It gets tricky. I deal with that question on the Children's Book Page.
 *My conclusion at this point:
Amazon uses the Mobi format and the rest use the Epub format. Since Amazon uses their own tracking number (called ASIN) and doesn't use the ISBN for tracking, I would use one ISBN, period.
Schools and pre-2008 e-readers use the epdf format. If an ebook producer asks for two ISBNs, that is why.
Whatever you decide, you will upload your edited manuscript, a cover image and at least one ISBN. The experts will do the rest. Once online, their is no annual charge to remain online. Either company charges around $150 if you want them to design a cover for you.
You are on the  Ebook page.


           Updated April 25, 2015