In addition to the missions in the Glacier Valley, there will be special BOSSs at specific times. After killing them, they will drop the flying skill book (currently the drop rate is very low)
Mutated Ice Demon Demon (1st floor of Glacier River Valley)
Flying Warrior: Secret Technique·Withered Spirit
Sword Master: Secret Technique·Secret Technique of Fangs·Butian
Wushuang Shadow Player: Secret Technique·Secret Technique of Devouring·Jue Shadow
Ice and Fire Walker: Secret Technique·Secret Technique of Extreme Ice·Shattering Cold Mystery·Scorching Spirit
Holy Light Messenger: Secret Technique·Resurrection
Mutated Ice Soul Mysterious Demon (Glacial Valley Level 2)
Flying Warrior: Secret Skills·Secret Skills of the Secret Blade·Secret Skills of the Rain Spear·Withered Spirit
Sword Controller: Secret Skills ·Secret Skill of Shield Dance·Bone Shattering
Private Shadow Man: Secret Skill·Secret Skill of Soul Lock·Secret Skill of Soul Needle·Dragon Throat
Ice and Fire Walker: Secret Skill·Return of God
Holy Light Messenger: Secret Skill: Leading Machine
Violent Feng Jingyun (3rd floor of Glacial Valley)
Flying Warrior: Secret Skill: Thrust
Sword Master Player: Secret Skill·Secret Skill of Soul Destruction·Ziqing
Private Shadow: Secret Skill·Secret Skill of Soul Needle·Dragon Slaying
Ice and Fire Walker: Secret Skill·Yanhuo
Holy Light Messenger: Secret Skill·Mystery of Compassion·Sacred Heart