Looking at the changes, it seems you want cover_w/h even when not get_cover or has_cover ? In what situations ?

OK, you could also have just moved the 2 lines just before the if not get_cover then break:

                bookinfo["cover_w"] = tonumber(row[num])
                bookinfo["cover_h"] = tonumber(row[num+1])
            if not get_cover then
                -- don't bother making a blitbuffer
                break
            end

to avoid the 3 string comparisons on each column if col == "pages" or col == "cover_w" or col == "cover_h" then.

Btw, does this all work fine if, started from an empty bookinfo cache, having Mosaic with text covers (so, I gues no cover fetch), getting some stuff cached, and then switching to a mode with cover images?

Yes, works fine with that case.