When enchanting an item it's best to use as many levels as possible for the best effects. Finding this level requires you to put the item multiple times in the enchanting table (certainly for higher levels).
This plugin saves you this time by making the third option in the enchanting table your max enchanting level. It looks at your current level and the maximum possible enchanting level (based on the number of bookcases) and changes the third option in the enchanting table to the minimum of these two values.
This mod only affects the third option of the enchanting table, nothing else.
Just put the MaxEnchanting.jar in your plugins folder, no configuration required.