In solo play you can add notes to it.
Also, I think you can click the notebook icon next to each course icon in the game lobby to add to them.
In multiplayer games it is read only.
**Edit
3 months of trying eh? I just found this in the FAQ's in about 13 seconds.>>>>>>>>>>>
A Course Notebook is a item where you can write and save notes about each hole on a WGT course. For Stroke Play couses you will have one page for "Fairway" notes and one page for "Green" notes. CTTH courses have only one page per hole. The notes you take will be saved and you can retrieve them whenever you are playing that hole.
If you own a Course Notebook you will see a small Notebook icon located to the right of your username when playing a game. Click on the icon to have the Notebook appear on your screen. You can only view your Notebook and not the Notebook of other players you are playing with.
You can enter and save notes into your Notebook during single-player practice, ranked or tournament games. In multi-player games you can view your notes but cannot enter new notes.