Born and raised in Alabama, Rosa Parks’ activism began in earnest at age 30, when she joined the Montgomery chapter of the NAACP and was elected secretary at her first meeting. Twelve years later, on December 1, 1955, on her way home from a long day of work as a department store seamstress, the bus driver asked her and three other black passengers to get up so that a single white man could sit down. Such a request was par for the course in highly segregated Montgomery. But this time around, Parks refused to budge. Arrested and fined $14 for violating Jim Crow laws, her treatment prompted an immediate boycott of Montgomery’s city buses, led by a young Martin Luther King Jr. Despite the threat of violence—local whites, for example, dynamited King’s house—the boycott lasted for more than a year, ending only when the U.S. Supreme Court declared segregated buses to be unconstitutional.

During the bus boycott, both Parks and her husband lost their jobs. Unable to find new employment and facing constant death threats, they moved briefly to Hampton, Virginia, before settling in Detroit, where they would spend the rest of their lives. After a few more years working as a seamstress, Parks was hired in 1965 as an administrative aide to Michigan congressman John Conyers Jr. Remaining politically active, she joined protests of such things as General Motors’ 1986 decision to close several plants. Parks died in 2005 at age 92, not long after being awarded both the Presidential Medal of Freedom and the Congressional Gold Medal. In a final tribute, she became the first woman to ever lie in state at the U.S. Capitol.

Over the course of her life, Parks saved thousands of photographs and documents, including letters from presidents, birthday and thank-you cards from schoolchildren, personal correspondence with family and friends, tax returns and even a handwritten recipe for “featherlite” pancakes (key ingredient: peanut butter). To the chagrin of historians, this sizeable archive languished unseen in a warehouse for nearly a decade owing to a legal dispute between her heirs and friends. Finally, in 2014, Howard G. Buffett, the son of billionaire investor Warren Buffett, acquired the archive for $4.5 million through his philanthropic foundation and gave it to the Library of Congress on a 10-year loan. Since then, the Library of Congress has worked to digitize the collection, announcing last week that it was now available online for public viewing. (At the conclusion of the loan, the digital files will stay at the LOC, even if the physical items move elsewhere.)

In one undated letter from the archive, Parks explains her reasoning for refusing to relinquish her bus seat on that fateful day in 1955. “I had been pushed around all my life and felt at this moment that I couldn’t take it anymore,” she writes. “When I asked the policeman why we had to be pushed around? He said he didn’t know. ‘The law is the law. You are under arrest.’ I didn’t resist.” Another manuscript contains her reflections on race relations in Montgomery, which she refers to as “the Cradle of the Confederacy, Heart of Dixie.” Lamenting the absurdity of the city’s racial customs, she points out that the airport had separate restrooms for “white ladies” and “colored women,” but that the planes themselves were integrated. “Night time integration and day time segregation makes this a very mixed up place,” she writes.

As her reputation grew, Parks met with numerous celebrities and world leaders, including boxer Muhammad Ali, author Maya Angelou, Vice President Al Gore and Pope John Paul II (all of whom are shown alongside her in archival photos). She also maintained a correspondence with King, who, in a 1957 postcard, declares, “I am thinking of you constantly.” Yet the archive equally shines in providing poignant family moments, such as a series of tender letters she penned to her husband during a brief time apart. “I miss you so very much and wish you were here,” she writes in one. “The weather is not cold now but you should be here to warm my feet by the time winter comes.”