Eagles Over the Kennebec

I have no idea about US tax law. However, I imagine the problem is in the valuation. The archived typescripts have never been sold, and thus there is no market value to ascribe to them, apart from their value as raw material. The situation is different with an artwork by a famous painter, as there are well understood valuation models, as well as a history of market prices for most of them. If the typescripts had been obtained from an auction, they would probably be valuable under the auction price, but they have never been sold.