Address Book App adds the letter "n" to my notes
I think this is a bug with iCloud. I started seeing the same behavior when I started using iCloud. When I turned iCloud off and went back to syncing over the cable, the issue of extra "n"s went away. 

Try backing up your address book (just in case) and turning off iCloud for a while. If that fixes it, you can make the choice on if you would rather have it on and deal with the "n"s or off and have to sync with the cable. 

Of course, this will also likely get fixed in future release as iCloud is tweaked