Excellent work.

Right off the bat it seems you drawing with your wrist more then your shoulder or elbow. Some of your lines appear to be jagged in nature rather than one smooth execution. Think back to the comic with the samurai swinging his sword. It is okay if lines overlap, try to stay loose. Learning this will not come over night. You will have to sketch thousands upon thousands of cubes. So dont get frustrated if it does not come right away.

Your foreshortening is excellent in the latter pages. That can be the hardest part sometimes. Well done on that.

Nit-picky thing try to use just one drawing utensil as you move through the exercises. It looks like you used a sharpie outlining the cubes. Not necessary. Use the same utensil and thicken up the lines. This is good to strengthen your ghosting techniques and add weight to lines.