There are a lot of books about drawing on the market so it's hard to separate the good ones from the bad ones.Andrew Loomis is an artist and an author and I found his books pretty helpful when it comes to drawing.Of course you can't learn to draw from books but they hold valuable advice although the most important thing is as always,practice.So practice an hour a day,30minutes a day,10 minutes a day it doesn't really matter,practice when you have time and don't force yourself if you don't feel like it,soon you will see improvement.So here is a list of books that I found helpful.
Books by Andrew Loomis:
Drawing the head and hands
Fun with a pencil
Books by Bridgman:
The human machine
Books from other authors:
Vernon Blake - The art and craft of drawing
Walter Crane - Line & Form
De Laszlo - Painting a portrait
Arthur Leighton Guptill - Sketching and rendering in pencil
Jean Paul Richter - The literary works of Leonardo da Vinci (Volume 1 and 2)
There are many more and if anyone wants me to send them the links to the E-Books just say
so in the comments.