"HW rulez" mod (add-on) for HoMM3.

The most optimal solution for the library in the Tower has long been proposed:

Rebuilding does not change the spells that were added to the slots after the library was built.

1) There is no need to ban the library.
2) It provides its own bonus and allows you to upgrade mages.
3) Rebuilding works in the Tower, just like in other cities (i.e., 5 level 1 slots, 4 level 2 slots, 3 level 3 slots, 2 level 4 slots, and 1 level 5 slot are rebuilt).

I hope that this is the version that will be implemented in the next Heroes & Might & Magic patch.