Q: iBooks doesn't read my epub files

Hi,

I have installed the latest version of ibooks on my iPad.

First of all I wanted to try if it works properly.

So I sent an e-book (epub format bought from a seller that is not Apple) by e-mail from my Mac. I recovered it from the mail (the e-book icon - the same as iBooks - was recognized as an ePub file) and it asked me if i wanted to open it with ibooks.

Well, the app said that the file was not in a proper format to be read.

This happened with other epub files.

Another app read those files perfectly (asked me only the authorization to read epub files inserting ID and PW for the Adobe account).

iBooks asked me nothing about the authorization...

Does it means that iBooks reads epub files bought only from Apple? In this case i won't be able to recover all my old e-books with this app.

Thank you.

iPad Air / iOS 7.1.2