Video walkthrough with English commentary!
As we leave the library at the Vatican, we can grab the Theft Report note inside the first tent just in front of us. This begins the Fieldwork side quest A Savage Discovery.
Head to the scaffolding area near the Sistine Chapel to meet and speak to the Professor. You must be wearing the Clerical disguise to speak with him.
After speaking with the professor, sneakily enter the restricted camp area west of the main courtyard. This can be done by climbing the scaffolding to the right of the camp entrance or sneaking under the buildings to the left of the entrance. Inside we need to find the professor's assistant, who is locked in a cell. Defeat the guard outside the cell and grab the key from the wall to free him.
Return to where we first met the Professor to get our next task. Then, infiltrate the excavation site and grab the Guard List note from the excavation site office. In the excavation site we can jump down to section F to find Giuseppe, beating him up and taking his cellar key.
The cellar is located down the alley to the right of the Post Office entrance. Enter the cellar to grab the artifact for the professor, defeat the enemy that tries to stop us, then meet the Professor on the first floor of the library to complete this quest. He will reward us with the Marshall College Exploration Guide Book.