The "open in iBooks" option does not appear

I've created an iBook that I do not want to publish to the iBook store. Instead, I have simply hosted and shared the .ibook file via Box.net.

I've tested the situation on a few iPads. When I download the file to an iPad, the "Open in iBooks" option does not appear. Instead, it either gives me alternative apps to open the file with (i.e. Dropbox or Evernote) or simply says "Open in..." but the button is not clickable.

For what it's worth, I've tested this on an iPad running iBooks 1.3.2 and iBooks 2.0 with the same results.