Success! I went through the whole process again but didn’t connect to my wi-fi. I borrowed a different book which I opened directly into ADE, went to Library, right clicked and saved to the connected Nook. Disengaged Nook from PC and it was available to read. I think the book I was concentrating on initially (i.e. the book my wife had actually wanted to read!) has got messed up in my several attempts to load it.
It’s great you have this sort of info here Terence, thanks for the help.