The Cave Twins

According to Wikipedia: "Lucy Fitch Perkins (12 July 1865 18 March 1937) was an American children's book author and illustrator, famous for writing the Twins series of books. Lucy Fitch was born in Maples, Indiana, the daughter of a factory owner and businessman. She studied at the Fine Arts School in Boston, and later taught art at the Pratt Institute in Brooklyn. She worked for a time as an illustrator with the Louis Prang & Company. She married architect Dwight Perkins in 1891; they lived for many years in Chicago. The Twins books featured stories on twin siblings from different nations or different periods in American history. Twenty-six volumes of the series appeared over two decades, from the 1910s through the 1930s: The Dutch Twins, The Japanese Twins, The Eskimo Twins, The Puritan Twins, The American Twins of the Revolution, etc."