Historian Theodore in the Library will give you a variety of quests looking for old books. Some of these are Lost Memories while others are stand alone quests or related to the Main Quest.
Historian's Request - Find Worn Book on 4th floor's Miniature Garden. Go to Passage of the Gardener from Miniature Garden 3, and to Room of the Gardener from there. Reward: ? xp, ? gold, Lost Memory, What is Alchemy?.
Lost Memory - Get Worn Book from 6th floor's Place of Dazzlement 2 at book cases. Reward: 400 xp, 400 gold, Lost Memory, The Tower and the Wisemen.
Wisemen's record - Get Worn book from 9th floor's Golden Workroom 4, located just north of the bottom entrance. Reward: 1000 xp, 500 gold, Lost Memory: The Wisemen's frustration.
Record of Betraying the Wisemen - Get 20 Dwarf's Ancient Books from Gnoll Berserkers on the 12th floor. Reward: ? xp, ? gold, Lost Memory, Betrayal of the Wiseman.
Sealed Record - Kill 30 Giant Octopussys through the new portal in the Library. Reward: ? xp, ? gold, Lost Memory, Research of defensive device.
The Last Weapon - Get Record of Ultimate Weapon. Located on 22nd Floor, Forest Of Truth 4. Reward: 10000 XP, 4500 gold, Lost Memory: The Last Weapon.
Record of Completion - Find all of the Lost Memories. Reward: 20000 XP, 10000 Gold, 2 skill points.