When curing a zombie villager, there is a chance to get a trade for 3 bookshelves for a price of 1 emerald, you can then trade with the same villager 1 book for 1 emerald, effectively turning one emerald into 9 if you break the book shelves into single books
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We're actually already tracking this issue in MC-153334, so I resolved and linked this ticket as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
If you haven't already, you might like to make use of the search feature in the future to see if the issue has already been reported.