Design History.org (Amazing resource, check it out.)

Leonardo Da Vinci Sketchbooks (Interactive book, British Museum site)
Manuscript Link

16th Century Book Can Be Read Six Different Ways (My Modern Met)

MC Escher

Artsy.net: What is Fauvism? (article with timeline and images)

Saul Dewitt on How to Be An Artist (Artsy Article)

How to be an Artist: Paul Klee (Line/color)

Piet Mondrian on How to be an Artist (Artsy Article)

Graphic Design 1954-2004: from Elaine Lustig Cohen

Typography Posters (Design your way.net)

Mythology:

6 Greek Myths You Should Know to Understand Art History (Artsy Article)

Japanese:

Japanese Graphic Design from the 1920-1930’s
(So beautiful. Inspiration .pdf)

Japanese Graphic Design

Vintage:
55 superb vintage posters  (topdesignmag.com)

Vintage Style Guides and Free Vector Graphics

Vintage Advertising (Pinterest Board)

Vintage Hotel Labels (Pinterest Board)

Free:

150,000 Botanical and Animal Illustrations Available for Free Download (Colossal Article) BioDiversityLibrary (How to download an image.)
https://about.biodiversitylibrary.org/ufaqs/how-do-i-download-an-image-from-flickr/

Cleveland Museum of Art Works Free

My Modern Met: 200 Vintage Posters from the Golden Age of Design Now Free

Metropolitan Museum of Art Now Offers Over 1,600 Free Art Books Online

Free Digital Archive Makes Over 50 Million Pieces of European Art Available Online

Free Images From Museums, Libraries, and More (Create Adobe: Blog Article)

You Can Now Use 375,000 Images from the Met Museum for Free