My thoughts about balancing villagers

I think that while villagers are definitely OP the way to nerf them is not to create a more tedious experience for the player to trade but to create other ways in the game to collect enchanted books. In my opinion, villagers should be primarily used in order to collect large quantities of books (such as on servers) while another feature should be added to collect smaller amount of books but in a more enjoyable way.

My suggestion is to be able to find unenchanted books of a specific type (whether this be for 1 type of book or a collection (such as protection type with the protection, fire protection, projectile protection and blast protection enchantments) in structures (this would also encourage exploration). These books could then be enchanted in an enchanting table to gain a random enchanted book of this type. These books could also have a much higher chance of getting the higher tear enchantments too (eg efficiency IV and V) in order to make this method of collecting books more favourable. This feature could also allow the addition for unique textures for the type of book. While this doesn't nerf villagers it offers another way to gain enchanted books (that many players will find more enjoyable) meaning that many the more casual players will likely use this method. 

In addition to this some of the trades should be further balanced and there are a few bugs (eg it is possible to buy glass, turn it into glass panes, and trade 1 for 1 emerald).