Japanese artist Nagai Hideyuki has created a fun series of drawings that span two sketchbooks propped at 90° to one another, and when viewed from the proper angle, give the illusion of continuous three dimensionality.
You can see a selection on his website and on his deviantART page.
There is a video on YouTube that gives a clearer picture of the relationship between the two sets of drawings.
[Via Visual News by way of Colossal and io9]