.zip to .epub files won't be open by iBooks

config : MacBook Air, El Capitan

.zip to .epub files won't open with iBooks, even with native .epub files.

I have created two tests :

TEST 1

1. Downloaded an .epub file from Gutenberg (no DRM), clicked on it == it opens iBooks

2. I manually changed extension to .zip, uncompressed the archive to look into its content

3. I manually compressed the folder back -- without editing then changed the .zip back to .epub

4. .epub file will NOT open

I have done this with several .epub repeatedly. Only visible difference is 5Kb more.

Permission-wise, I have then tried before step 3 :

a) not changing any file permissions (won't open)

b) changing all files permissions to read+write (won't open)

The reason I did this test is because of TEST 2

1. I wanted to manually create an epub by reproducing the file structure, .zip the content and change to .epub

2. When I do this, it wouldn't open even if I followed the right specs