I use Atlas of Human Anatomy for the Artist to learn human anatomy. It’s a bit dense but he goes into why things fit the way they do, which is important.
I recommend Atlas of Human Anatomy for The Artist by Stephen Rogers Peck . This book is one of the go-to anatomy books for many artists.
It has both anatomical illustrations and very informative written sections containing useful details and notes of proportion, as well as illustrated "shorthand" types of shapes for many of the body's shapes.
As another data point, I bought Atlas of Human Anatomy for the Artist, but found it's at a way higher level of detail than what I would have found most useful. If you are trying to do super-realistic drawings where you can see every muscle in their foot, this is a great book, but it's not ideal for basic anatomy.
Atlas of Human Anatomy for the Artist by Stephen Rogers Peck is nice. It covers the bones, muscles, and surface anatomy (fat, hair, veins, etc.). It also has sections ( smaller than the three above though) on proportion, equilibrium and locomotion, differences of age, sex, and race, and facial expression. It's 279 pages, will take a while to read, and can be exhausting if you're not interested. http://www.amazon.com/Atlas-Anatomy-Artist-Stephen-Rogers/dp/0195030958
Depends on what she already has, but if she doesn't have any of the following, she might like it.
edit: You can probably find lower prices than from Amazon, and you can get aquarium filters to cut up for the stipple sponges.
It gives basic structure and is less "dry" than some of the academic anatomy books. (I love my anatomy books, don't mistake—I think my favorite was this one: https://smile.amazon.com/Atlas-Anatomy-Artist-Stephen-Rogers/dp/0195030958/ )
Loomis was a game-changer for me, though. It's been considered a classic for so long for a reason. There are a lot of newer anatomy books and I don't have them all, and I'm sure many of them are excellent. I think Loomis is a welcome addition to any artists' library and for me, was essential.
other people have put it forward but yah you are pushing details too early before establishing a decent general form, the cartoon you're trying to use as reference has a very squared off face shape, but at the same time you could draw a skull under it and all the muscle and then fat and skin forms, same goes for the hair, you're better served by adding the hair on top of a skull and face form even if it's not a "realistic" skull
also not sure what your history is but it's 100% better to learn to model / sculpt realistic forms before you do cartoony ones, and also REALLY important to have a good knowledge of all the forms "underneath" what you're sculpting
if you want some cheaper anatomy books I recommend peck's anatomy for technical and much more accurate knowledge https://www.amazon.com/Atlas-Anatomy-Artist-Stephen-Rogers/dp/0195030958
and Bridgeman for more simplified but form focused https://www.amazon.com/Constructive-Anatomy-Dover-Artists/dp/0486211045
also, finish projects get some second opinions and move on! nothing beats practice and trying the same exercise multiple times!
O Proko είναι καλός δάσκαλος για τα πρώτα σου βήματα και διδάσκει συνδυάζοντας την γνώση των πιο καταξιωμένων στον χώρο, πχ George Bridgman, Gottfried bammes, Loomis, Michael hampton κτλπ. Εάν μπορέσεις να βρείς και αυτό εδώ το βιβλίο ακόμα καλύτερα. ;)
I think I stumbled on this one via Proko and I like it.
https://www.amazon.com/Atlas-Anatomy-Artist-Stephen-Rogers/dp/0195030958/
The best practice you can do is life drawing while paying very close attention to proportions. Actually studying anatomy wouldn't hurt either. There are plenty of books out there for artists that are technical but not painfully so. This is the book I used when in school.
Now for a critique:
This one has a pretty nice overall design. Though he doesn't really seem to be engaged to the objects he is holding. If you push his pose a little more with his shoulders towards his resting hand and drew in an actual twist between the spine and the hips it would sell the pose better. If you are going for human anatomy, you generally have the right silhouette but the details don't really match. For example, the abdomen area isn't human. The top part is too low to be pecs and too large to be the abs. Also the legs appear to be pulled out of socket a bit. The screen left hand with the gun much farther ahead would look better if it was also in the silhouette. Without it, you either make the gun look ridiculously large which makes it look impossible to hold up, especially in that pose, or that the gun is the hand. If the latter is the case, then the pose should be adjusted so that is clear. Last up, the tilt of the head (based off the eyes) isn't followed up with a tilt in the rest of the skull which gives it that Sloth from the Goonies look. All of those things could be legit design choices, but without the pose information to back it up, they make it look unfinished.
Your girl image:
This is not something I would show. It is really unfinished and unevenly unfinished at that. As it is, the silhouette looks wrong anatomy wise. Your torso is too long, you are missing landmarks and/or muscle groups around the neck/scapula that if they were there would make your characters neck disappear and ridiculously thin. Her legs are also very stubby while the far one doesn't seem to be attached at the correct location for that pose. Her arms, especially the "finished arm" are both proportioned like baby arms. Her face looks good for that style, but without the cranium it also looks pretty squashed. Generally, the proportions on the whole drawing are all over the place.
In both images, finding any type of pose reference would really help. Knowing the anatomy on top and using simpler shapes can help you adjust the poses when they aren't working quite as well visually.
I'm currently an illustration student and I found that studying anatomy greatly improved my figure drawing. The more you study the human form, the better you will get. I highly recommend this book. My professor had us copying anatomy drawings from this and it really helps to familiarize yourself with what is under the skin creating those bumps and crevices that you see on the surface.
Also, I would pay attention to where the model is shifting their weight while standing. Generally, the suprasternal notch (notch between collar bones) is directly above the foot that the model has shifted his/her weight to. This is their center of balance. This helps your figures look more stable/not falling over.
For seated figures, it is really important to first check how tall the model is sitting vs. how wide they are. Try measuring with your thumb and pencil (or a wooden dowel) like this. I would also recommend drawing bigger and with charcoal. You're off to a great start! Keep practicing :)
These are great notes, but they are no substitute for having/studying anatomy references yourself and observing models closely to draw your own conclusions.
Even professional illustrators (especially professional illustrators!) continually build their library reference materials and keep them within reach to help with various projects. Every artist can point you to their personal favorites.
Here are two of mine if you like the content of this link:
Atlas of Human Anatomy for the Artist by Stephen Peck. This one features lots of insightful notes about rendering various bones and muscles AND goes into detail about the origins/insertions of all superficially relevant muscle groups.
http://www.amazon.com/Atlas-Human-Anatomy-Artist-Galaxy/dp/0195030958
Strength Training Anatomy by Frederic Delavier. It has very clear and helpful renderings of muscles and is organized by regions of the body. This book is actually targeted towards bodybuilders but it's a great resource for artists, too. Just remember that not everybody is built like the people in the book :P http://www.amazon.com/Strength-Training-Anatomy-Frederic-Delavier/dp/0736063684/ref=dp_ob_title_bk
The notes posted in the OP's link are born from the studying of these kinds of books AND the observation of styles and rendering decisions of other artists and it's a good idea for all growing and aspiring artists (myself included) to always take notes of interesting observations from everywhere and everything.