Brenda Mangan here, not Donna. But I can say drone notes are usually what the base, or lowest, note of any accompanying cord would be. Or sometime the second note of any accompanying cord. So if you have sheet music, you can go by that. If you want to drone one note all the way through a song it would usually be the note of what key the song is in. So, you see it can vary, that would be why in her book, Donna "suggests' drones to use. Think about how the bag pipes sound, the quintessential in music with drones; all three, and therefore any one, of the individual notes of the drone pipes sounds good with the tune.
Happy Bowing, ~Brenda