Villager Apprentice
Villagers don't teach you. You teach them. Replaces the slot machine rolling of villager trades with a more adventerous system.
An untaught villager
Empty hand shows what a villager knows at a glance. This novice librarian knows nothing yet, 0 of 1 slots used and they'll never roll enchanted book trades on their own.
The teaching preview
Sneak while holding an enchanted book and the HUD tells you what a click would do before you commit. This librarian would learn Density IV and sell it for 14 emeralds + 1 book, using 1 of their 1 knowledge slots.
Merging duplicates
A duplicate of a book they already know merges anvil-style. Density IV + IV becomes Density V for 10 XP levels, and the HUD quotes the price before you click.
Free upgrade
Found a better book later? Teaching a higher level upgrades the existing trade in place, no cost. The arrow means the trade is about to go up.