There is 4 type of heroes and each can learn different books for each type
Attacks can learn Warmonger, Marksmen, Frenzy, Drill Sergeant, Valor, Berserker
Defense can learn Entrenchment, Backlash, Guardian Will, Vengeance, Dragon Skin, Fortitude
Agility can learn Shadow Master, Chaos Bringer Illusionist, Swift Mover, Athleticism, Pulverizer
Balance can learn only two skills Acclaim and Healer