These books were written around the WWII-era by a man called William Andrew Loomis. He was a very accomplished and widely published artist at the time, but what he's currently more well-known for is a series of art books he created during that time. These books have stood the test of time and their contents are still commonly used and recommended to art students of a wide range of skill levels, and if you're looking to improve your drawing skills, particulary faces and the human body, these books are a good place to start.
The major strength of his books is that they're not a step-by-step guide or tutorial on how to draw a specific picture, portrait or pose. Many art books fall into this category, but Loomis' books go beyond that, and teach the actual process of creating art. On top of that, he comes across as very genuine and supportive, and it's obvious from his writing that this is a man who deeply loves his craft.
For a long time his books were out of print and practically unattainable. This is when initiatives such as "Save Loomis" were started, to make sure the books remain accessible. If you wanted to get a hardcopy of an original you had to be either rich or you were shit out of luck - some becoming collector's items entering the $100 -$400 price range. During the last three years however, Titan Publishing attained the copyrights to these books and as of writing this, republished all his books but one of them. They can now be found on Amazon for around $25 each.
The Save Loomis initiative (and many like it) still exist, so these books aren't that hard to find if you searched for them online. However, I can't in good conscience post the links here or recommend you download them, and I heartily recommend you buy a hardcopy of these books if it's at all possible for you. Getting a hardcopy of the books will do its contents justice more than a digital copy will, and you support both the publisher and Loomis' family.
That said, let's talk about the books themselves.
Fun with a Pencil: how everybody can easily learn how to draw
This book deals with the basics of learning to draw, and is useful for everyone of novice to intermediate skill. If you're looking to get better at drawing the human figure, this book covers damn-near everything.
The book starts off with simple drawing exercises where cartoony faces (which he calls "blooks") are created by combining spheres and other three-dimensional objects, but in later chapters we are taught how this applies to realistic portraiture as well. A similar method is used to "build up" the entire human body, starting off at the frame that resembles the human skeleton. Loomis alternates between making caricatures and realistic people and bodies, showing that this method is useful for both.
In later chapters he also covers facial expressions, the hands, folds of clothing, perspective, and shadow.
What else can you ask for?
This book deals with perspective, light, and how those two interact with eachother. It's very comprehensive: this book deals with the perspective and light on buildings, landscapes, reflections in water and mirrors, portraits, and bodies (both nude and clothed). Many of the techniques from Fun with a Pencil are reintroduced, and it shows how having a solid grasp of how light works can be used to draw natural light from heart.
This book is more advanced than the previous one, and already assumes you have some degree of knowledge and experience under the belt (at least the "building"-exercises from the first book).
Figure drawing for all it's worth
This is pretty much the Bible of figure drawing. If you're serious about becoming a character artist of any kind (or you're not, but you just want to be able to draw decent-looking humans), this is one book you need to own. It's also fairly accessible for beginners.
In "Figure drawing for all it's worth", Loomis further elaborates on human anatomy and structure, such as the bones, the muscles and the difference between men and women. The models he uses aren't all static, he dedicates a lot of time to the figure in action; how the body changes when it's performing, how weight distribution and balance shift, and how you can translate these movements on a canvas or piece of paper to create something that's not just anatomically or structurally correct, but dynamic as well. He covers the human figure in perspective, as well as light and shading on nude and clothed figures. He dedicates some time to the ears, eyes, and lips. Lastly, he covers the hands and feet.
This book deals with lines, composition, color and illustration. It covers illustration from beginning to end - starting off with a single line to indicate the line of action, or to set the composition. (Though, for this book you should already have some basic drawing skills.)
In addition to dealing with drawing techniques, this book also contains information on generating ideas and communicating a story with your art; some of the most valuable assets of good illustration that are often overlooked in other educational art books or art courses. The last chapter deals with the different fields of commercial illustration.
Drawing the head & hands
This book consists of five parts, with the first four covering the heads of men, women, children and infants. He spends some time dealing with ears, eyes, noses and mouths, but his main focus is on the shape and proportions on the head. Only the last part deals with the hands, but despite that he still manages to put a lot of information in those last few pages.
The Eye of the Painter, and the elements of beauty
This book deals with painting, and contains many examples of other people's work. Loomis emphasizes the importance of creativity, vision and idealism in each of the works of these artists.
This is a pretty advanced book however, dealing with more advanced concepts of creating a composition than in his previous books. This is also the only book on this list that's in the public domain, so it can be downloaded for free. Finding a .pdf of this book isn't hard, but finding one that's in color is a bit more tricky. (Seeing as one of the subjects Loomis covers is color theory, that might be useful.)