When I export as an iBook widget and drop it into iBooks Author, I preview the book on my iPad and when the page loads, the text line spacing shifts downward when the page is loaded. This does not happen when I preview on my iMac.
I've used several different fonts and it doesn't make a difference.
Also, when I use the bold version of the font Avenir Next Condensed, the text shifts from normal to italic.
I attached a video.
shift.mov (2.5 MB)