Acording to the wiki the odds of catching an enchanted book is 1.6% with luck of the sea III. Meaning for every hundred catches, you would catch 1-2 enchanted books on average. Unbreaking and mending do not affect the rate at which enchanted books are caught at all. Lure make it take less long to catch something, and Luck of the sea increases the chance that what you will catch will be valuable. Hope this helps.

As mentioned in my longer answer in your other question:

Spawning rules depend on the position of a player. Mobs cannot spawn if there are any players in a radius of 128 block around the spawning spot in question. That is why you need to be at your farm for it to work at all.

You normally put your afk spot above the farm, since that brings you further away from the ground and possibly the caves below. But as long as you are more than 128 block above ground or all places that aren't in your farm, but closer than 128 blocks to you are lit up properly it does not matter if your afk spot is above/below or next to your farm.