Hi Zod, and thanks again. I have checked the 'expand' folder (though it isn't really a folder, is it? at least, not when I look inside the tablet from my Windows 10 laptop) and it's not there or elsewhere.
I too am using the paid version of Moon but I will give something else a go.
[edit - later]
I downloaded the free version of Bookavi from Mantano, and this manages access to books stored on the external SD card without any problems. So the issue does seem to be at least partly with Moon and not Android or my Lenovo tablet.
If I can work out how to upgrade it, I might go for the paid version of Bookavi to get rid of the very annoying adverts. In other respects it looks like a good piece of software.
[edit - later still]
I think I may now have reconciled what Zod has been saying to what I actually see on my tablet. Instead of calling the SD card "m_external_SD", it is called by some no doubt proprietary internal name, 2785-OEEA. So Moon can in fact find it via / > storage > 2785-OEEA > Books.
Hurrah. I hadn't thought of trying that rather forbiddingly-titled location. Oddly though, it also includes books that are on the internal storage of the tablet. One of the wonders of Android/Linux no doubt.
Thanks once again to Zod and all you other helpful readers.