Amantius Allectus' diary can be found inside his house in the Imperial City, specifically in the southeast corner of the Temple District. To get the diary, you need to enter his house, which is locked but can be lockpicked. Once inside, look to the right to find a reading desk on the first floor; the diary is inside this desk

. This diary is part of the Thieves Guild quest "May the Best Thief Win," where you are tasked with stealing it without harming Amantius Allectus. During the quest, Amantius is usually asleep upstairs, making it easier to steal the diary undetected

. In summary:

• Location: Amantius Allectus' House, Temple District, Imperial City
• How to get it: Lockpick the door, then open the desk on the first floor to find the diary
• Quest: "May the Best Thief Win" in The Elder Scrolls IV: Oblivion

This is the primary and reliable method to find Amantius Allectus' diary in the game