You've come a long way already since the start of this sketchbook. The newest life drawings seem very promising. I like the look on your ink drawings, especially this and the newest one and the pencil hatchings on the last page looked good too.
Along with the life drawing you could try out some anatomy books to help out with the proportions and constructions and everything. Loomis and Bridgeman are my favourites but any system for building the head and body so that the features find their places and sizes would do good for your referenced sketches too. Keep these coming!