The best way to open .notebook files, on basically any computer, is to use the proprietary SMART Technologies Notebook and SMART Technologies Notebook Express software that is available for Windows, Mac, and Linux systems.
The basic version of this software is available for free and you can download it from here.
A word of caution here is that if you do receive a .notebook file, and if you think that renaming the extension will change the behaviour of the file, unfortunately, it will not work, so please refrain from trying that out, as you can risk jeopardizing the file as a whole.
