The correct formatting style of the word 'epub'

Hello all,

 

I am an editor working on a manuscript where the author is using the word / term epub. On the internet, I see this written out as:

 
epub
ePub
EPUB
ePUB
 

So, what is the official way this term is supposed to be written? Especially in a more technical book.

 

Thank you for taking the time to help me with this.

 

David Puthuff

Editing by David

www.editingbydavid.com

david@editingbydavid.com