I agree with Evildisco about the faces, especially in anything prior to post #9 - it more or less looks like the same haunted man over and over again ~ that said I really like everything from post #9 and forward, if it's a speedy evolution, or just that your subject matter changed I'm not sure. For while earlier I found your shading top notch, but your anatomy troublesome, at least if it was meant to portray different types of people/characters - then comers the rabbit of #9, the bug man of #10 and the monstrosities of 12# and #15, and changed all that, and I really like em.
This is a very fascinating sketchbook. Your pencil work and cross-hatching is very topnotch. Real nice work! Not too bad on the digi pieces but they are coming off as alittle flat. Think more about your lighting first and form. If you jump into the colors head first it will mess you up. I've done it far too many times speaking myself. If your working in photoshop you might like this thread- http://www.conceptart.org/forums/showthread.php?t=47859
By the way, are you a fan of elephant man? Those two last pieces got me thinking of him.