Your sketchbook shows real progress; it was great seeing you work on nailing those forms/contours in your line drawings. I also picked up a few pointers/sparks of inspiration about my own process from reading this thread. Thanks!
My only piece of advice would be for your monochromatic studies - and that is to really experiment with your value range. See just how dark and how light you can paint things before they start to look wrong. I'm only just getting to this myself, but have found it to be another useful step on the way to believability.