Some of the classic art books are just that: classic! There are, however, some real gems amongst them.
Of the several online libraries that have titles available for download my favourite is the Internet Archive because they offer books in PDF format among others and don’t tease with a bunch of books that are just “snippet view” as do Google Books.
Internet Archive
Sketching and rendering in pencil (1922) by Arthur Guptill
French Art Classic and Contemporary Painting and Sculpture by W. C. (William Crary) Brownell (1851-1928)
Landscape painting (1909) by Birge Harrison
Modern painting by George Moore (1852-1933)
A Text-Book of the History of Painting by John Charles Van Dyke (1856-1932)
A treatise on painting by Cennino Cennini (15th century)
On drawing and painting by Denman Waldo Ross (1853-1935)
The practice of oil painting and of drawing as associated with it (1911) by Solomon J Solomon
Sir Joshua Reynolds’s discourses on art (1891) edited by Edward Gilpin Johnson
Google Books
When you go to Google Books do a search but don’t get too excited with the list that comes up. At the top left of the results of your search there will be a drop-down box “Books showing”. Select “Full View” – to get the ones worth bothering with. Once you have the book you want you can read online or look to the top right for ways to save the file – eg PDF…
An Analysis of Beauty by William Hogarth (1810)
Lectures on Painting Delivered at the Royal Academy by Henry Fuseli
The art of drawing in perspective by James Ferguson (1778) – definitely a classic…
Project Gutenburg
Line and Form by Walter Crane
Pictorial Composition and the Critical Judgment of Pictures by Henry Rankin Poore
Outdoor Sketching by Francis Hopkinson Smith
The Practice and Science of Drawing by Harold Speed
The Mind of the Artist – Thoughts and Sayings of Painters and Sculptors on Their Art edited by Cicely Margaret Powell Binyon
Concerning the Spiritual in Art by Wassily Kandinsky (1866-1944) translated by Michael Sadleir (1888-1957)
Lectures on Landscape delivered at Oxford in Lent Term, 1871 by John Ruskin (1819-1900)