Hi there! Starting a sketchbook like this is the perfect way to help yourself get better. There are lots of friendly, helpful people on this site and it's a great way to look back and view your own progress.
It looks like you are off to a great start! Probably the most important thing for you to do at this stage is practice those proportions and shapes. There are tons of photo's on the net that are useful for figure drawing. Don't worry about doing a ton of shading or trying to make things look realistic at first. Those first few figure drawings you have are a great example! Just getting use to the forms and proportions are really important. And from what you have posted, you already have a good handle on that! It's all about mileage man. If you look at the really great artist on here, they draw nearly every day, without fail. Because they know that every single stroke they make takes them a step further from where they were yesterday.
Sorry for rambling so much. Hope some of that helps though. Keep up the great work and keep posting!
Great anatomy studies, maybe you should do some 30s ones? just to understand better the outline of the human body?
Also, you could use some more observation on the references.
I know it's not easy, but keep going ^_^
i kinda like the 30s ones more than the other ones .. you can feel the movements of the reference through just a few lines, even in the ones that are quite unfinished, but if you think that's too hard for your current level, try to do some 5~10 minutes drawings and then a few 30s ones just to warm up or something.
well, i can surely see some development going on there, mostly in that 20 minutes and in the last 2 minutes one.