To increase a hero's skill level, you need blue skill books for regular skills and red skill books for their ultimate skill.
It takes 15 blue/red skill books to go from Level 1 to Level 2.
It takes 35 blue/red skill books to go from Level 2 to Level 3.
It takes 75 blue/red skill books to go from Level 3 to Level 4.
In total, it takes 125 skill books to level up a skill all the way.
