The description says that it was "designed as a compliment to Calibre", which can probably also be taken as an indication that BookONO is using a fair amount of code from calibre. But in spite of the shared code it only vaguely resembles calibre:
BookONO also differs from calibre in that it:
does not attempt to put all your books in its own library or change their names. Where Calibre attempts to take complete control over your books, BookONO gives you control over your books.
This ebook manager has also moved beyond calibre in at least one way. It has a web browser built in, and you can also use BookONO to convert any web page to PDF and store in BookONO.
I have not tried this app myself; I'm going to wait for a beta release. Have you tried it, and if so what did you think?