You cannot open an epub in InDesign.
If you want to edit an existing epub, InDesign is not the tool —as Bob Levine said—.
If you want to edit "extensively" an existing epub 3 with fixed layout you'd better commit suicide if you don't have the original InDesign files and then remake the job with them to export the epub from anew.
But, if you want just to edit a fluid layout epub —or just an epub 2—, you may use any coding tool able to edit code, complex code. If you have a slight knowledge of regular expressions (regexp), so much the better.
oXygen XML, Calibre "modify book", jEdit, and many other pieces of similar software could be the right choice.
Hope it helps
Gustavo (posting from Spain)
As Gustavo said, oXygen is a very great soft to open .epub files without dezipping them [directly modify and save them]. I use it too (for .idml files) when I want to make very heavy corrections totally impossible directly in InDesign.
Thanks to all - The Adobe description of InDesign raved about its new features viz a viz fixed designs.
What I have is an epub ebook created in 2010 format and technology. It has been live with Kindle and Nook for years.
I need to extensively revise it, and switch from the old fluid layout (which works poorly on different devices) to a fixed layout for Apple devides - which InDesign claims is done easily.
I have been editing it directly with Calibre in HTML, but wanted to do the layout work more visually and easily.
Why does the INDesign just say "YOU CAN'T EDIT EXISTING EPUB HERE!"
The ability to CREATE certain files is not the same as the ability edit them.
True - but why make such a great tool if it cannot also be used to edit existing files? Calibre, Word, and Coda, etc. all do that with ease.
Since I have all the txt and images I need (mostly), why can't I just start this as a "new" project in InDesign? Will it not open and insert simple text and image files and let me position as desired? Then I could save the whole thing as a fixed epub.
Sure. You could do that but that’s not what you have.
You have a whole bunch of HTML files.
What was used to create this thing in the first place?
I sent it out to some outfit that scanned the original print version (LonelyPlanet/Pisces Books) and converted the output into into the files I need to submit to Nook and Kindle.
So now I have the nook and kindle versions, and I think I can pretty easily work with those.
But now I want to do a fixed version for ibooks which I hear can be viewed on all their devices and rendered properly. That's where InDesign seemed to be a ready-made tool, but isn't.
You are incorrect in your assessment and your expectations are unreasonable.
InDesign is quite capable of creating beautiful fixed layout epubs.
I suggest a bit more study on the subject.
OK - I'm open to that - but the help manual I have seen is not that user friendly, at least not to the Adobe-Newbie.
I don't see much in the way of straightforward advice on the basics - like how to begin writing and inserting images on a page, organize pages into chapters, etc.
This is a high level professional application.
You need training and good book or two if you expect to use it successfully.
For your use, I would suggest Sandee Cohen and Diane Burns new book on digital publishing with InDesign: http://amzn.to/1Cxb0GX
You would likely benefit from Sandee’s Quick Start Guide: http://amzn.to/17QkeVd
Finally, some quality time on lynda.com. This link will get you a 10 day trial. Take advantage of it: http://bit.ly/1xHYd0i
Ok - thanks for the help.