[*Note, May 1, 2010: Sorry, most of the items below dating from later than 1923 have recently been made unavailable for download. On the other hand there are very many older books that have been added since this post was made - just use the search function on the site]
...Barcsay, Perard, Moses artistic anatomies; Rockwell, Dore, Kollwitz biographies, ebook editions of old books illustrated by Wyeth, Pyle, Rackham, Crane ..; letters of Michelangelo, Van Gogh, .. ; Vasari's Lives complete in ten volumes; ebooks by Bridgman, Panofsky, Berenson, Kenneth Clark, Whistler, Ruskin ...; Leonardo notebooks illustrated ..; and tons of old illustrated art books.
Archive.org has been around for years, but there has been a HUGE influx of new art titles in the last few months. To search, make sure you select "texts" in the second box. To download, click "HTTP" and save to disc either the pdf or the (often smaller) DjVu file. Both consist of page images with linked text files, which means that the text is searchable.
Very hi-res scans can be obtained by downloading the zip archive of jp2 files present in the HTTP folders for many of the books. I use the freeware programme Irfanview to batch convert the jp2 files to jpgs.
Just some of the juiciest tips of the iceberg (to mix metaphors):
Bridgman, George B., 1939. The Human Machine*.
Bridgman, George B., 1920. Constructive anatomy
Blake, Vernon, 1927. The Art And Craft Of Drawing*
Crane, Walter, 1914. Line & Form.
De Laszlo, 1934. Painting a Portrait*.
Eastlake, Sir Charles Lock. Methods And Materials Of Painting Of The Great Schools And Masters
Guptill, Arthur Leighton,1922. Sketching and Rendering in Pencil.
Holmes, C. J. 1920. Notes on the science of picture-making.
Poore, Henry Rankin, 1903. Pictorial composition and the critical judgment of pictures.
Ross, Denman Waldo, 1907. A theory of pure design; harmony, balance, rhythm.
Ruskin, John. The elements of drawing (1920 edition).
Richter, Jean Paul, The literary works of Leonardo da Vinci (Volume 1 and 2).
The same illustrated edition of Leonardo notebooks that was reprinted by Dover.
Stone, Irving And Jean, 1962. I, Michelangelo, Sculptor (Michelangelo's Letters).
Whistler, James McNeill, 1909. The gentle art of making enemies, as pleasingly exemplified in many instances, wherein the serious ones of this earth, carefully exasperated, have been prettily spurred on to unseemliness and indiscretion, while overcome by an undue sense of right.
Ludovici, Anthony Mario, 1912 - The letters of a post-impressionist; being the familiar correspondence of Vincent van Gogh.
Gauguin, Paul, 1919. Noa Noa. Translated from the French by O.F. Theis.
Vasari, Giorgio. Lives of the most eminent painters, sculptors & architects (complete in ten volumes).
Clark, Kenneth, 1952. Leonardo Da Vinci An Account Of His Development As An Artist*.
Hind, Charles Lewis. Drawings of Leonardo da Vinci.
Mount, Charles Merrill, 1955. John Singer Sargent - A Biography*.
Marillier, H. C. 1912. The Early Work of Aubrey Beardsley.
The later work of Aubrey Beardsley.
Quennell, Peter, 1955. Hogarth's Progress*.
Clapp, 1916 - Jacopo Carucci da Pontormo, his life and work.
Panofsky, Erwin, 1955. The Life and Art of Albrecht Durer*.
The classic study of Durer
Strange, Edward Fairbrother, 1906. Hokusai, the old man mad with painting.
Clark, Kenneth. The Romantic Rebellion.
Clark, Kenneth, 1949. Landscape into Art*.
Panofsky, Erwin, 1955. Meaning In The Visual Arts - Papers In and On Art History*.
10 works illustrated by NC Wyeth .
30+ works illustrated by Howard Pyle (+ 10 on Gutenberg).
28 works by, or illustrated by Walter Crane.
EDIT: Check below the dotted line for new recommendations
Maginnis, Charles. Pen Drawing - An Illustrated Treatise
http://www.gutenberg.org/etext/17502 (4.9 MB zip)
Sullivan, Edmund J. 1921. The Art Of Illustration.
Sullivan, Edmund J. 1922. Line - an art study
Books illustrated by Arthur Rackham
Goblin Market (scanned by chaosrocks)
Vernon Jones, V. S.1916. Æsop's fables : a new translation. (Rackham, Arthur, illustrator)
Gates, Eleanor, 1907. Good-night (Buenas noches).
Morley, Christopher, 1922. Where the blue begins.
Ibsen, Henrik, [19--?]. Peer Gynt: a dramatic poem.
Stephens, James, 1920. Irish fairy tales.
Fitz-Gerald, S. J. Adair (Shafto Justin Adair),1896. The Zankiwank & the Bletherwitch.
Evans, C. S., 1920. The sleeping beauty.
Robinson, John Beverley, 1914. Architectural Composition (found by Farvus)
Other architectural highlights include 50+ elaborately illustrated volumes of the journal known successively as Architecture, American architect and architecture, and Architectural record:
plus numerous volumes both on the history of architecture in general, and on specific styles including Egyptian, Greek, Byzantine, Norman, Gothic, Romanesque, Medieval, Muslim, Indian ... Just use the search button.
Books on the imagination (found by chronologic; reviewed by armando here)
Ribot, Th. 1906. Essay on the creative imagination.
Rhoades, James, 1908. The training of the imagination.
Dow, Arthur Wesley, 1913. Composition; a series of exercises in art structure for the use of students and teachers.
Fashion (from Farvus)
Ellsworth, Evelyn Peters, 1917. Textiles and costume design
Traphagen, Ethel, 1918. Costume design and illustration
More Artists and their works
Heermann, Norbert, 1918. Frank Duveneck
Roof, Katharine Metcalf, 1917. The life and art of William Merritt Chase
Cox, Kenyon, 1914. Winslow Homer
Inness, George Jr,, 1917. Life, Art and Letters of George Inness.
Hartmann, Sadakichi, 1910. The Whistler book; a monograph of the life and position in art of James McNeill Whistler
Macfall, Haldane, [1906?] Whistler
Furst, Herbert, 1920. Frank Brangwyn, R.A (woodcuts)
Sparrow, Walter Shaw, 1915. A Book of Bridges (illustr. Brangwyn)
Brooklyn Museum, 1921 - Historical paintings of the Slavic nations (Mucha's Slav Epic)
Books with illustrations by N.C. Wyeth
The throwback : a romance of the Southwest (1906)
Arizona nights (c1907)
Whispering Smith (1908 [c1906)
The song of Hiawatha (c1908 )
Susanna and Sue (c1909)
The long roll (1911)
Treasure Island (c1911)
Cease firing (1912)
The sampo; a wonder tale of the old North ([c1912])
Blackfeet Indian stories (c1913)
Letters of a woman homesteader (1914)
The drums of the 47th (c1914)
Nan of Music Mountain (1916)
The mysterious stranger : a romance (1916)
The black arrow : a tale of the two Roses ([c1916])
Robin Hood (1917)
The last of the Mohicans : a narrative of 1757 (1919)
Robinson Crusoe (1920)
The courtship of Miles Standish (1920)
Rip Van Winkle (c1921)
Poems of American patriotism (c1922)
Vandemark's folly (c1922)
Chapman's American Drawing Book (added by patdzon)
Chapman, J.G. (John Gadsby), 1847. The American drawing-book: a manual for the amateur, and basis of study for the professional artist: especially adapted to the use of public and private schools, as well as home instruction.
http://books.google.com/books?id=NdAEAAAAYAAJ (1847 edn: 168 pp.)
http://digital.library.pitt.edu/cgi-...ew=toc;c=nietz (1847 edn: 168 pp.)
http://books.google.com/books?id=BoQEAAAAYAAJ (1858 edn: 304 pp.)
http://digicoll.library.wisc.edu/cgi....I0001&isize=M (1870 edn: 320 pp)
Loomis, Andrew, 1951. Successful Drawing.
http://acid.noobgrinder.com/Loomis/A...%20Drawing.rar (direct download link)
Storey, G. A. The Theory and Practice of Perspective.
http://www.gutenberg.org/files/20165/20165-h.zip (direct download link)
Watson, Ernest W., 1955. How To Use Creative Perspective*.
http://www.archive.org/details/howto...ative036957mbp (poor scans)
Longfellow, William Pitt Preble, 1901. Applied perspective, for architects and painters.
Miller, Leslie William, 1887. The essentials of perspective, with illustrations drawn by the author.
Great collection of scholarly articles on history of perspective (Kim H. Veltman):
edit by davi: