Help CenterMending, as talked about in a few other guides, is an enchantment that will repair your enchanted item with the experience you gain upon killing mobs. This enchantment is extremely useful for your high-value items, essentially preventing it from being destroyed, or at the very least, prolonging it's use for substantially.
Mending enchanted books can be found through Librarians, a villager job. In our case, we're going to need a village, an unemployed villager, and a decent amount of luck.
Fake Disclaimer: I am not responsible for any Geneva Convention violations you may or may not make during this process out of frustration or malice against the villager population.
Step 1: The Unemployed Villager
We'll need a Villager who's currently lacking a job. Doesn't matter who, what, when, where, or why, just needs to be unemployed and accepting of a new job. Note, this will not include Nitwits.
Step 2: The Lectern
Stolen from another village? Crafted? Doesn't matter how you got it, but you'll also need one of these as well.
Don't know how to craft one? Well, here you go:
Now that you have your lectern, it's time to join the two together
Step 3: Employment
Place the Lectern down near your unemployed Villager. You should see their outfit change with a few green sparkles around them if done right. This is now your Librarian. Check their trades.
In our case, I got a Frost Walker book. Not what we needed, but this is fine. Just break the Lectern. This will cause the Librarian to lose their job and return to being unemployed. Simply place the Lectern back down afterwards to set them as a Librarian again and check their trades. Repeat until you have a trade available for the Mending (or other enchanted) book you need.
This may take a while. Just remember not to murder your villager out of frustration. Doing so may not cause any immediate aftermath, but killing enough and remaining within the same village may make the Iron Golems not so happy with your presence.