HoMM3 HD is a mod (add-on) for the game Heroes of Might and Magic 3.

IMHO. Library +1 spell - only as an addition (non-rebuildable), in case the "rebuild library slot" option is disabled. And all magic slots are rebuildable - with the option to rebuild the library enabled. I would vote for this solution. The first case would be used in most games, and the second case would be used in games where one of the players starts with a Tower.