Adam’s Diary, first published in 1904, was illustrated by Frederick Strothmann (1879 – 1958). An artist for Harper’s, Strothmann illustrated other books by Mark Twain, including the story The Celebrated Jumping Frog of Calaveras County.
The illustrations in Eve’s Diary, published in 1906, are by Lester Ralph (1877 – 1927). Twain wanted an artist with a more refined approach for Eve’s Diary. Ralph was an accomplished painter as well as an illustrator, so he was given the job.