Learning spells is a skill for scholars and unlocking secrets.

You need to remember this!

Although it's obvious, I'll add that you can also learn spells from the "Tome of Darkness" in the same way. I think it doesn't matter where the spell is used in battle by an enemy hero, as long as it's used, it will be learned.
I'm playing through the original missions in "Tribes of the East" and I've maxed out Agrael's dark magic skills, and all the top-tier spells are already available by the third mission. There's a hidden artifact shop in the bottom left corner of the map; that's where I bought the "Tome of Darkness" and gave it to the enemy.
It's a bit of a hassle, and not really necessary, but... just for fun, I like to max out my heroes before moving on to the next mission.

And I'll add that it's not necessary for a secondary hero to win the battle; it's enough for the enemy hero to use the spells. Then you can escape and hire the hero again in the tavern.