Why won't downloaded book add to my library?

It's been a while since I used ADE but am pretty sure when I downloaded books before they went into a folder on my PC called My Digital Editions as part of the install, where all the books I'd added to the library were stored.

Have just installed ADE on new laptop and found the book opens and I can read it within ADE but the Add To Library option is greyed out and therefore the book isn't in that folder.  When I click on the downloaded book, it simply launches ADE to read it there, which I don't really want to as I prefer another app on another more reader friendly device than my laptop. 

The downloaded book is a free ARC from Netgalley but I'm not sure this is significant as I've had many of them previously which I could add to the library.  Any ideas?