I own those, I would describe them as semi-hardcover, with fake leather wrapped around some stiff but still pretty flexible material. The text is also so small as to be difficult to read in all but ideal lighting.

Tbh I have a worn UK first edition of one volume (stored for now, I think it was TT). Beautiful fold out map, but the cover was cheap and flimsy. I think my latter US 2nd edition was better manufactured, not sure if that’s a postwar UK problem or not.