During her extended field trips to regions of Spain from the 1920s to the late 1940s, commissioned by the Hispanic Society of America (HSA), she took thousands of photographs and accompanying notes on Spanish life and people.

[4] In 1921, working as an interior designer in New York City, Anderson was hired by the Hispanic Society of America (HSA) at the recommendation of Clarence White himself.

[4] There, she started as one of a group of young women in their twenties, who had been hired without specific training for their eventual specialization: Similar to Elizabeth du Gué Trapier, Beatrice Gilman Proske, Alice Wilson Frothingham, Florence Lewis May, and Eleanor Sherman Font, Anderson acquired considerable expertise in the history of Spanish art and civilization through her lifelong activities for the HSA.

[6][7] Between 1923 and 1930, Anderson made five long trips to Spain, taking documentary photographs and collecting related information for the HSA, mainly in Galicia, Asturias, León, Extremadura and Andalusia.

In great detail, she commented on the ethnographic background, referring to local dress and lifestyles, regional Spanish history and literature of Sorolla's paintings.

"[15] Her last book, Hispanic costume, 1480-1530,[16] covers the period of the last decades before the Conquest of Granada and Christopher Columbus' first voyage to the Americas, both in 1492, until after the coronation of Charles V as Holy Roman Emperor.

Also, she wanted "to enable theater designers to proceed in something of the assured freedom with which Renaissance tailors, furriers and shoemakers cut and sewed to please the taste of their clients.

[16]: vii-ix  In his 1981 review, Joseph R. Jones of the University of Kentucky called Anderson's work an "occasional book by an old-fashioned connoisseur-scholar, whose character is as interesting as the subject matter.

[4] The Ruth Anderson Collection, which has served as a source for publications on Spanish historical customs, folklore and dress, forms an important part of the archives of the Hispanic Society's Museum and Library.

[21] In a Spanish study on Anderson's representation of the past and on contemporary popular culture in digital media, her work was referred to as an "exceptional collection of images taken in the 1920s in different parts of Galicia, portraying our people, customs and traditions, an ethnographic legacy of incalculable value.

Dating back almost 100 years, Anderson's pictures and the recreated scenes of rural life showed, among others, milkmaids gathered in Santiago de Compostela's Rúa Nova at dawn, women carrying ceramic pots to market, craftsmen selling wooden clogs and people in traditional raincoats made of straw.

[29][30] In the 2019 Spanish graphic novel Los cuentos de la niebla (Tales of the fog), some of Anderson's photos of Galicia were reproduced as sources of local life in the 1930s.

Representations of the past and digital reinterpretations", Maria Jesús Pena Castro noted that Spanish women were shown by Anderson in active roles as workers, owners of capital and competent decision-makers.

[34] In Lenaghan's view, Anderson saw her images as mere visual documents of Spain, but the attention they have found after her death through exhibitions and publications have lent another quality to them.

As she shot routine events with a close attention to detail, yet framed them in carefully constructed compositions, she drew on both documentary and artistic traditions.

Anderson's work and the entire project of assembling such an archive thus present another chapter in the long running debate of what Spain is with, in this case, a contribution offered up by an admiring outsider.