a few months ago (before 1.13) and today, I was trying to get a villager to offer me a mending book. according to everything I've seen so far, the books offered should be randomly enchanted with equal weights assigned to each item, and it was taking me so long to get to mending (like, Minecraft weeks) that I started taking stats, and I've seen every book except mending so far in the chart, most of them (including Infinity, which I use to gauge against Mending, considering how valuable both of them are) more than four times each. I'm pretty sure it's not a statistical anomaly. I'm using a setup where I take away a librarian's workstation and put it back down to refresh his trades every few seconds. I've been running it for probably a cumulative five or six hours.