Thanks for the info re syncing. However, I am not sure that that is what I am looking for? (No doubt being clueless here!) I don't want to sync them, I want simply to be able to download one or two of my audiobooks from my Books app on my computer, to my Books app on my phone or iPad. And I do not want copies on other devices (my computer especially) to be deleted when I delete a copy on one of my devices. The syncing instructions seem to suggest that that would happen. Or does it mean merely that the copy in the iCloud is deleted so it would not appear in the Books app on other devices, but that that does not affect the original copy I have in a folder physically on my computer?
It is very strange that the Books app on computer and phone seem to say that audiobooks including free ones will appear in the way ebooks including free ones do, yet whilst the epubs do appear, the mp3 books do not.