Relation to Milo
Though it is unknown as to exactly what Milo wrote in the journal it can be assumed that, due to be Observer Symbol etched into its front cover, it contains some (if not all) of Milo's knowledge about the Observer, Collective, and/or the Slender Man. We first see the Journal in Submission #5 of the series as Milo writes in it. Noah claims that Milo told him it contained 'observations', but he states that Milo never let him see it's contents.
In Case Findings, Noah shows us the Journal and tells us the little information and significance of it that he is aware of as of yet. He claims that he found a key in the soles of a pair of loafers that he gave to Milo before his death, but demonstrates that the key is useless and does not open Milo's Journal.