This is what I'd call art school, but without the hefty tuition. If you work through these books, you'll have a very solid foundation and be well on your way to being a professional
Essential study for learning the human figure
Bridgman's Complete Guide to Drawing from Life
This thing is a treasure trove of knowledge. It's a textbook worth of information on how to draw everything and anything. Expensive, but worthwhile!
How to Draw: Drawing and Sketching Objects and Environments from Your Imagination
Once you know how to draw, this one will help you with giving things three dimensional form by applying the laws of light
How to Render: The Fundamentals of Light, Shadow and Reflectivity
James Gurney is a living master and this book contains a wealth of information
Color and Light: A Guide for the Realist Painter Volume 2
The Loomis method will take you very far in drawing heads
Drawing the Head and Hands
Good book if you want to make comics
How To Draw Comics The Marvel Way
If you're interested in ink drawing, this is a great primer on how to handle the medium
Pen and Ink Drawing: A Simple Guide
Great book on how to tell a story with your art
Framed Ink: Drawing and Composition for Visual Storytellers
This isn't a how to guide on art, but a how to guide on life. This will help you find your creative voice and solve problems you didn't even know you had
The Artist's Way: 30th Anniversary Edition
Perspective is a useful tool for any artist
Perspective Made Easy
If you're into fantasy, this will help you make wondrous creatures
Science of Creature Design: Understanding Animal Anatomy
Another great fantasy book from a modern master
Imaginative Realism: How to Paint What Doesn't Exist Volume 1
For the animal lovers out there
The Art of Animal Drawing: Construction, Action Analysis, Caricature
Great break down of the “why” behind how comics work
Understanding Comics
Great for those wanting to become concept artists
Designing Creatures and Characters: How to Build an Artist's Portfolio for Video Games, Film, Animation and More
