Successfully complete the following levels to unlock the corresponding secret book.

Attack at Cedarleaf: Zhai's Past
The Wizard's Tower: Khelben's Letter
The Jungles of Chult: Illius' Youth
The Yuan-Ti Temple: Nesme's Fall
Stand at Mithral Hall: Cireka's Vow Drizzt's Journal
Into the Underdark: Ygorl And The Dragon