So version┬áv3.0.0fc9774 of the Nook for Mac application has an occasional but quite annoying bug in that, on launching, it hangs at the splash screen and will not respond to input, though according to Activity Monitor it is not Not Responding, so it hasn’t crashed as such.

The workaround (that I picked up from this thread) is to right-click the grey area of the application’s workspace, then click ‘Reload’ on the menu that appears. This should reload the splash screen and then get past it to loading your library. Hope this helps others.