There is some really substantial improvement seen throughout this sketchbook. Well done!
One tip I do have for you is to use bigger brushes - almost so that it's a little annoying to use. This forces you do consider your strokes and your intentions a lot more and in return you get a much better "workout" as well as a much cleaner looking painting. My own "mentor", so to speak, always told me "the less strokes you need to describe something, the better". Try to analyze an object and identify what is the bare minimum of information needed to make sure that form reads the way you want it to. It doesn't mean you can't have textures or all that good stuff. It's just when it comes down to the major forms, really being critical about how you describe those forms will in time give you a much better understanding of them.
Hope that little ramble was helpful.. I needed to remind myself of this as well