All eBooks are closed systems by definition.
Their aim is to create a reliable shell for commercial distribution of information.
With a restricted access even for viewing the content's source code, it's hard to imagine that they allow somebody to edit it.

The TuneText does!
TuneText creatively "implants" special Editor inside HTML content.
This means, - inside eBook!

TuneText turns your eBook into safe and highly interactive info-product!
There is no access to HTML source code!
The best thing is, - original content always remains intact!
Only user's changes to the content are saved outside.
Of course, there are some limits as for what TuneText readers are allowed to do. But the full list of TuneText features is quite impressive!

Read more on How It Works.
Underlying Concepts are described in the main TuneText info-product (download it now).