I only learned about the Librarians having three books total like 2 weeks ago. Before that I wasted so much diamonds and levels on enchanting and repairing my tools.

For the past week I've been breeding villagers (not with an actual breeder, but by just killing the unwanted ones out of my self-made village) in search of Librarians with good books, preferably the desired mending.

Just a minute ago one of my white jacketed babies grew to an adult and his first book was MENDING! (for 18) I was so excited, I immediately placed him in a minecart and pushed him to his own little spot I had made for him. When I got him there, I unlocked his second book, ALSO MENDING! (this time for 24)
Before this I hadn't encountered a Librarian with two of the same sale so I was confused. Anyways, I unlocked the third book and it was YET ANOTHER MENDING!!! (and for only 12 emeralds!)

As I said, I'm relatively new to the whole villager sale thing but this seemed like a rare thing to happen, so I decided to share it to the people who care about stuff like this 🙂