The secret diary of Matthew DeFoe is a book found in the Matthew's bedroom in DeFoe Manor in 5 Days A Stranger. It contains entries written by Matthew between 1814 and 1821. The text of the book is reproduced below.

June 28th, 1814
Today is my eighth birthday. Nanny said I would soon be a big strong man like my father. I wanted to see him today, but he wouldn't come out of his study all day. Nanny says he always gets cross on this day, but it's not because of me.
August 5th, 1817
I found a new friend. He is behind the door in the kitchen and he doesn't have a name. He likes me to sit and talk to him, but father gets very angry when he finds me doing this. I asked father who the boy behind the door was, but he told me there was no boy behind the door and it was just a silly fantasy.
May 24th, 1820
I showed father the painting I just finished. He said it was quite good, which is the best thing he has ever said. While he was in the good mood, I asked if I could go and look behind the door in the kitchen. He pretended not to hear me.
June 28th, 1821
My father has done a terrible thing. All this time he pretended there was no boy behind the door, and now this. There is blood all over the kitchen floor. I will do what I can. Then we can be a family together and be happy.