Hello Frank:
Very nice portrait. I think the head area is the strongest. Nice lighting on the form; you captured the hair very nicely. You have expressed a pretty good understanding of value and how light rolls over the form to create highlight and shadow.
Now for the critique:
Her eyes look appropriately set back into her head, though the eyebrows might be just a little dark. She appears to have fair hair, like blonde or light brown - yes? The eyebrows don't appear to match her hair values. Very nice lost edges around the ear and points where the hair meets the background and the opposite side of her shirt. The shadowed nasal-labial crease (the little crease that goes from the edges of your nose around your mouth) is too dark and makes the area look too deeply set. Further, if you added more values between the highlight on the cheek and the shadow of the crease it will make the cheek turn better and look more round. Nice reflected light on the underside of the nostrils. I might bring some higher values in to the highlighted area of the forehead, again to make it look rounder and turn better. The cast shadow from her head that falls on her shoulder will be darker where it meets the hair and lighten as it falls away from the head. The edge will be sharper closer and soften as it falls away too.
Her lower half needs some work. Her arms appear to terminate too high and her bustline also appears too high. Remember, the arms are cylinders, so they need to appear round. Make your value transitions longer and softer to make the form of the arm look more round. The bump you have represented is the Acromion-clavicle joint, the spot where the clavicle protrudes out of the shoulder. You need more surrounding values otherwise it looks like a spot.
The torso and the shirt are cylinders too. As the shirt rolls into her arm, it will get darker. Let the reflected light on her arm define the edge. I might put some higher values into the shirt as well to make it look like a white shirt, but that may just be the scan. Nice turn on the collar of the shirt. It looks pooched out.
From a composition standpoint, she is slightly off-center, which is good. Again, this may just be the scan, but I would put a couple more inches above her head. Where to end the form is a difficult choice. You have to take it past the bustline for it to not make the figure look cut off...good choice. I might make it more abstract closer to the bottom, which is the place to leave strokes and rough shaded areas. Finally, IMO, you should not sign on the form itself as it messes with the illusion. Sign down on the lower corner of the page - again, my opinion.
I hope that helps! Again, a nice piece.