It'd be extremely complicated to bend and twist the hero skill leveling system to adjust the spells in a spellbook using dummy spells (or whatever way this could be done).
Honestly, it, sounds like you're way off in the deep end here, with editor experience at a point where you are still figuring out the basics.
I'd recommend that you rethink how you want this to work, and then maybe find someone to help you. Alternatively, you adjust your expectations to be way lower and find a way to work with that.
If you want this to work, you would need to settle on a simple leveling mechanic for the spells in the spellbook. The easiest way I can think of, is to level up all of the spells in the spellbook every time that the hero levels up, but this is already some fairly difficult trigger programming.
But on another note. Units can have 7 spells shown in the standard UI. Do you really need more than 7 spells per unit in your rpg map? If some of them are passive, you could always hide them, and add their effect to the description of an actively used spell, leaving you with 7 slots for actively used spells. That's more than Diablo 3.