[ProDigit]

Does only a ZIP program work, or can I also use RAR or 7Z compression?

Can I open an epub file with eg: winzip (or other free program), and replace the files containing the book's text, with my book's text files?

I've opened several epub files and see a lot of useless data there.
Most showing conversion done with calibre, which I see as the new Microsoft, filling up text lines and unnecessary lines of code which only enlarge the book's size.

Some of them defining font types, but I don't really need special font types. The standard font types will do just fine.

Concerning a book's size, you'd recommend keeping it at 128kB or smaller, and dividing large books into chapters instead (eg: have chapter files)?

I'm kind of reverse engineering an epub, to see which data can be eliminated, to still give the same results.