Villager Apprentice

In vanilla, a player often has to break and replace a lectern again and again until a villager's random roll finally produces a worthwhile trade. This mod replaces that slot machine with a system where a villager teaches what a player brings. Enchanted books are found in the world, through fishing, dungeon chests and raids, and are then given to a librarian. The book is consumed and the villager learns it permanently, and a copy can be sold from the trade screen afterwards. Untaught librarians offer only paper and bookshelf trades.

Teaching is done by sneaking and right-clicking a villager while holding a teachable item. Ordinary right-clicking still opens the trading screen, so a villager never learns anything by mistake. A villager refuses an item when it is the wrong profession, when its slots already know that trade, or when the gear is damaged, and a refused item is kept by the player. Holding sneak and looking at a villager shows a small line of icons under the crosshair that names what it would learn, what the trade costs and which slot the item takes. Everything else about villagers stays vanilla, including the cheap trades.