Thank you, Picas. The trouble is that I normally avoid connecting my phone directly to iTunes on my computer because I want to avoid duplication. And downloading them directly to my phone through the iBooks app involves paying for them again, as I have discovered.
None of this is the end of the world, but since the books are supposed to appear in iCloud, I should like to find out why they are not there. I had already found and read the article for which you have kindly provided the link before I posted here, but it assumes that the books have got into iCloud — and they haven't.
I could, of course, load them manually into iCloud Drive. But, as I say, I should like to find out why something that seems tobe taken for granted in the literature has not happened.