Preview Grayed Out (disabled) in Toolbar and Menubar.

I am running iBooks Author 2.2 under Yosemite 10.10.3 on a 2011 Mac Mini.

In the past I have successfully used the preview function with my iPad 4, and with iBooks for Mac. However, today (after trying to switch to previewing only current section instead of entire book) the preview function stopped working. It is grayed out on both the Toolbar and the Menubar.

I have tried resetting preferences as before (default: Preview entire book) but have had no success. Any ideas?