You have a good eye for detail and values and color also. I think the most useful thing that you could do is work on your edges. At present they are kinda rugged, you need to clear them out at least in te focus areas. There are good techniques for that on http://ctrlpaint.com, be sure to check them out. A mandatory one is masking.
To work on the smootness of your strokes, be sure to do a lot of quick sketches, so that you can train your arm and wrist to move more fluidly. That's it for now, keep it up. Very interesting sketchbook.