Cavador's diary was a diary kept by the Mechanist Brother Cavador and is a pick up-able item from Thief II.
It appears he started to keep the current version when he was transferred to The Lost City in order to do archaeological work by order of Karras.
While happy with Karras and the option given to him to explore the area, he was a little annoyed that Karras was only interested in such small things, as the Masks and the Cultivators.[1]
The Diary can be found in Cavador's room in Site 5 and is a bonus objective in the mission 'Kidnap'.