This is the first of the free art book posts. I have found that there are many very good art books in the public domain. I will try and list links to them as I get the chance.
The first book is The practice of oil painting and of drawing as associated with it By Solomon Joseph Solomon. This book is one of the better books on the classical painting style that’s in the public domain. You can download it at the Internet Archive .
Here is a painting by the author.