H-hello! I'm still new to critiquing artworks on dA so I'll try my best to get at everything about this artwork. If I make a mistake please let me know ; A ;" ------
WHAT I LOVE Well, I definitely think you have good control over the colors you use and put them to great use while shading. Nothing is too shiny and you definitely made some nice contrasts, especially with the hair. The pose fits well and the background has a nice simple effect and impact to the over all artwork.
WHAT NEEDS WORK Okay I'm just going to try to elaborate as best as I can on this. Composition needs work. Although it is a simple concept I think the Rule of Thirds can easily be applied to this one. With my visualization of the lines for the Rule of Thirds, one of the main focus points would be Miku's breasts. The other would be the knee on the right. I'm not saying it's bad if you wanted the focus to be those two points, but it definitely changes what people naturally see first if the artwork was printed in front of them. And since it's art book work, you may want to consider that in your future works for it. --- Another thing I can see is the anatomy. It's good and definitely isn't terrible from what I see. One of the very first things I notice is the hand on the far left. It's easy to spot the mistake since it's not close to her body. The palm area of the hand is too bulky and wide while her fingers are too short. The thumb also sticks out too. It's too short. Consider this--- The thumb nail should end right before the first bone for the pointer finger ends. You got that aspect right on the other hand on the right side. However that hand still has an awkward feel to it. The fingers shouldn't be so curved with that positioning and should have a more angular feel to it.
Now for the legs, another thing I notice, is that the legs aren't the same length. Regardless of the the fact that one is supposed to be farther behind the other, you should utilize some curved line that makes the perspective of the legs correct. For the reference line I'm talking about, you would start from the left leg's need and make a quick stroke towards the right. When you have your curve, the knee of the other leg should hit that line. You can do the same for the feet too as I see it looks a little awkward.
WHAT I RECOMMEND
Here's something you don't have to follow but it's best that you do for better results. I'm not sure if you already do this but, before you go and draw the clothes in, draw out the anatomy! It's easier to spot the mistakes before you notice it too late! I've always had problems like such... I usually notice it once I start doing lineart which is a good time to catch any mistake. Also, if you're not too sure about the pose, just get a friend or someone else to look over the anatomy for you. It's good to have secondary eyes looking at it so they can catch things you don't notice. ------
Other than that, great artwork and keep it up! Please realize that I'm not making any sort of attack on your artwork and I'm only here to state unbiased ideas of the artwork. If you need any further explanations or red lines for me to actually point out what I mean, then please feel free to contact me through notes :'D