The EPUB Proof is in the iBooks pudding (hint: it's in the Advanced Menu)
EPUB is supported in both digital-rights managed (DRM) methods for proprietary bookstores, like Apple's iBookstore, as well as an unencrypted format that can be read on almost every device and in every bit of software (including iBooks)--except Kindle.
Because of that broad support, it's a reasonable format to distribute anything in, and Pages '09 (version 4) and Pages 5 both offer an export option. You can create a document in Pages, InDesign, and other software, and then export to PDF and EPUB for different methods of distribution or to different purposes. (I just finished a self-published book using InDesign in just this way.)
But the standard use of EPUB is for a reflowable document, like a sophisticated Web page. EPUB isn't a monolithic file format, as much as a specification. In fact, an EPUB "file" is a zipped folder of specialized HTML documents, images, and other information that can be inspected and modified. This is quite useful for tweaking CSS (Cascading Style Sheets) styles that are used for Web pages and EPUB books to control type, paragraph formatting, image wrap-around, and much more.
While the scope of CSS is well beyond a brief article--there are books about it, and it's a constantly evolving standard--if you already know how to tweak HTML or CSS, you can fix up an EPUB. But how do you preview the results
Apple offers Book Proofer, a Mac app you can only download when you're a registered iBookstore vendor through its iTunes Connect portal, which can interactively preview changes in an EPUB's underlying files on a Mac and on attached iOS devices. However, Book Proofer abruptly stopped working with Yosemite without warning.
The functionality was shifted into iBooks for OS X through a slightly hidden option. Select iBooks > Preferences, click the Advanced button, and check Enable Advanced Menu in Menu Bar. Now you'll have an Advanced menu. But before you get started, you have to prep your EPUB file.
Having gone that far, now you can return to iBooks and select Advanced > Add EPUB to Library as Proof. In my testing, the book appears initially at the end of the list of books in your Library, even if you have Sort By > Most Recent set in the pop-up menu at the Library's upper-right.
The EPUB appears with a Proof label superimposed over its upper-right corner. Double-click it, and the proof opens, and a Devices button appears in the upper-right of the book file in garish purple. Click that, and you can select any attached iOS device that has the iBooks app installed and open frontmost. Select a device, and the book will copied over. (At the moment, I cannot get this to work from either of my main computers with an iPhone 6. Baffling. But colleagues report on success without any burps along the way.)
As you make changes in the EPUB book, every time you save a file, the proofed version in iBooks in OS X and iOS will updated, allowing you a not-quite-live, but pretty interactive editing environment.