Inside the notebook the various monsters, sub-monsters, and all the items they drop are listed. But not only that, story elements have all been recorded and the notes even detail how each member of his team were levelled up and when. It could practically be used as a thorough strategy guide to get through the game.
According to the guy who discovered the notebook, his grandfather picked up this note-taking habit during World War II while serving as a radio operator at a meteorological observatory. He then carried it over to anything he did, including playing Dragon Quest V, it seems.
You can see the images from inside the notebook below.