Successfully complete the following levels to unlock the corresponding picture in the gallery:

War in Damara - Battlefield picture
Gemspark - Concept Art
Attack at Cedarleaf - Cedarleaf picture
The Wizard's Tower - Wizards Tower picture
The Jungles of Chult - Chult Jungle picture
The Yuan-Ti Temple - Chult Temple picture
Stand at Mithral Hall - Mithral picture
Into the Underdark - Githyanki Outpost picture
The Dragon's Lair - Dragon picture
The Lord of Chaos - Limbo picture

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