Quick note from mike:
I personally believe the greatest enemy to artists are barriers of the mind. At least, that has been the case with me. Upon graduating animation school I had two different Disney animators tell me independently that I "drew like a two-year-old" and "needed to go back to grade school for drawing." Eight months later, I was a professional concept artist because I had found an instructor (Rad Sechrist, below) that was able to teach like I thought.
What I DID learn in school, and from Rad Sechrist, was how to learn, and that has served me better than any lesson I was every taught. Change the way you think, perceive, and learn, and your art will improve quickly. Here are resources that helped me (in no particular order).
** I believe that instagram and watching other's art in short videos is probably your #1 resource nowadays for learning art. nevertheless, us old fashioned people used books and they are still better than instagram at some things **
Drawn to life Volumes 1 & 2 by walt stanchfield
These books will teach you to gesture draw, which means you will be able to capture the "soul" or "story" of a drawing very quickly without erasing. While many people aren't interesting in drawing cartoons, I feel gesture drawing is a fundamental skill for any artist interested in observing life, as life does not hold still and must be sketched - and designed - quickly.
Every artist should own these books and look through them often.
The weatherly guide to drawing animals by joe weatherly
Although this book is about animals, it really should be called "Crash course in the principles of drawing and the harsh realities of sketchbooking from life." I feel it is the world's most succinct text on the fundamental principles of drawing. If you memorize this book it will be to your benefit.
Every artist should own this book and look through it often.
The art of animal character design with David coleman volume 1
This book is a little more specialized toward character design, so for artists less interested in character design, you can skip this book. Again although it is about animals, and although the written portions are very brief, it has all the "meat" of character design principles that you will need to be familiar with.
Only artists that are interested in character design or animals will probably like this book. To be honest, some good googling will probably get you most of the info in this book, but at $10 on amazon the book is a steal. I bought it for $40 new and it was worth every penny.
Rad Sechrist's "How to" Blog
Rad (yes, that's his name) is a mechanical engineer turned storyboard artist at Dreamworks animation. As such, he has a unique perspective on how to learn to draw. And he made a blog about it. The blog's current posts are more work oriented, but his older posts from 2009 are worth pure gold and are what opened my mind about how to learn to draw. I believe every artist has to find people that "speak" their language. Rad speaks my language and was the key I needed to figure out cartoons post-graduating from animation school.
I feel his blog should be required reading for every artist and required STUDY for any animator or story artist. It will teach you how to learn.
100 Tuesday Tips by Griz & Norm
These are golden nuggets of information that are basically an upgrade of Walt Stanchfield's books on gesture drawing. When you graduate from that class, you go to this one. Each page is PACKED with important goodness and reminders on the basics.
This book will most likely appeal to story artists, cartoonists, or anyone that wants to draw people beautifully and quickly and compose them into scenes that tell a story.