3M do privacy filters for screens that cut down the viewable angle using a form of "louvre" technology (if I remember correctly). Not sure it would work with standard written notes. Some of the older ones might.
Another possibililty I remember from the "TV Times" quiz board game is a form of obfuscation, 2 colours, text is light blue, obfuscating colour was red "scribbles" type of pattern, which could be filtered by a red see-through plastic sheet. Not very convenient or subtle though.
The best suggestion I can come up with is a notebook with a verticle spine which you do not open all the way, which will offer you a modicum of privacy for your notes from the sides and lean back whilst you read them. Think like a card player, and if anyone gives you a hard time remind them of the need for confidentiality.
Hope this helps, and let me know if you find a good answer.