The spirit of the trade seems to be "two written books" but it is in effect "two identical written books".

This issue is unique from MC-30415 as that issue was specifically about dividing identical items into two slots not completing a trade, whereas this report is regarding what should be accepted as two specifically unique items to effect the intended trade.

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Further discussion: the trade as it is makes it essentially useless. To get two identical copies of a written book, one must first make an original, then make TWO copies of that original in order to generate two identical books, since not even the original will stack with it's own copy. It's an arduous process no player would ever ordinarily carry out. It simply violates the spirit of the trade, which appears to say, I'll take two written books for an emerald.

This could be resolved by simply adding another trading recipe which accepts any two written books.