Shift + left click on Enchanted book goes on first addend instead second addend in repair interface

This is because you can also merge enchanted books, and rename them. There's no bug here.

@Dhranios We should be able to rename enchanted books or any item added on second addend slot too if the first addend slot is empty.

Still, you can merge books too, the first item selected is simply though of as the item you want to merge things into, not as consumed item. This sounds much more like a change request than a bug to me.