Well, if you actually mean mediaeval (handmade books), very very few were produced on anything but skin, which of course looks very different to paper. The Chinese made rag paper ages before the Euros did, and the Egyptians and to some extent other Med. peoples made a sort of paper from papyrus, and I think I remember reading that the Aztecs had experimented, but even Gutenberg printed his first stuff on skin (of course it was cos he was trying to trick people into thinking it was all handwork).
Honestly, I'd have to be shown a certified-no-kidding-old fiddle label that had gone grotty from the atmosphere alone before I'd believe it. Dirty from dust, yes. Stained from spills, yes. Mildew, yes. Insect leavings, yes. But from normal domestic atmosphere? I can't think of any process that would produce discoloration. I even did a quick google to check whether there was something I'd missed out all these years apropos old paper going yellow. No mention. But if you have some cites I'd be very eager to read them.