I have been doing some experiments making iBooks for a large museum tour.
Did not work very well.
My tour is 1.5 gb and when exporting it from iBooks author it just starts loading on iPad and then quits after 5 sec.
To find out the limt I removed panoramas and found that 1.2 gb still did not work but at 884 mb it worked fine.
Apple says that 2 BG is maximum but it maybe that the problem is that the tour is one single 15 GB widget.
Unfortunatelly I also found another problem. Images in HTML text popups does not work.
I also have some problem with the distorted arrow hotspots I use on the floor, they do not work in my last tour even if the code is identical from the tour I made 2 years ago.
The standard arrows made by the toureditor seems to work fine.