How about adding Hero Inventory Ability before adding tomes to them and after that removing the ability?
mine works, i proof it, there's a item that increases max HP permanent, so you have to use the ability of it, something like hitpoints bonus ability... just create custom ability of it, change it's HP added to 50 and everytime you want to add HP just add the ability to the unit via trigger, its pretty simple
@graystuff11
And what will hapen when the level of the ability reaches it's maximum?
That is not the solution.