Musk, 50, captioned the tweet with the name of the book, written by Barbara Tuchman in 1962, along with the entire first paragraph of the book.

'The muffled tongue of Big Ben tolled nine by the clock as the cortege left the palace, but on history's clock it was sunset, and the sun of the old world was setting in a dying blaze of splendor never to be seen again,' so ends the paragraph. 

Tuchman's book, centered on the first month of the Great War, was an immediate bestseller and earned her a Pulitzer Prize for general nonfiction. 

President John F. Kennedy was so impressed with it that he gave a copy to each member of his cabinet and some of his top military advisors and told them to read it.  

Musk followed up the Guns of August tweet with another literary reference, referencing Lord of the Rings, writing, 'Nine rings for mortal men.'