Two-sided box; what's the deal?

[ozarktroutbum]

I am trying to figure out how the "high" and "low" side is achieved. I am guessing they are the same height, but just differ in the amount of thickness.

But then again I could be wrong.

I am trying to get into building them. I have made a few rough ones so far. Also, does anyone know of any good books that could help me out?

Also, how do most folks do their paddles/lids? Belt sander? rasp?