Stax is a portable perspective drawing tool, simply place the ruler anywhere on your page, position the metal disc under your vanishing point, and the ruler will snap down with a magnet and rotate around that point. 
 
Stax supports 1-, 2-, and 3-point perspective drawing and includes sample project tutorials for each method. Any sketchbook size can be used with the use of the extension rulers, making it a great way to lay down a foundation for perspective drawing.