When you revive a zombie villager and give them a librarian job with an enchanted book as a first trade once you get them a high enough level they can have a trade that you can use to get free emeralds and xp. For example, I had a bookshelf trade of 2 emeralds and then right below it let me trade one book for one emerald. So basically If I have only 2 emeralds I can buy a bookshelf then break it turning it into three books, selling them for emeralds giving me 3, and repeating to get infinite xp for villagers trading infinite books, and bookshelves and infinite emeralds.