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It would have been all but impossible to write Passing Strange: A Gilded Age Tale of Love and Deception Across the Color Line without access to online resources, particularly to the digitized census records that proved such a rich source of information for my story. In the past, historians who wanted to find someone in these records would have to scroll through roll after roll of microfilm, looking at the handwritten records compiled by the census agents who assembled their data household by household as they walked down a street. If you didn't know a person's exact address, it could be all but impossible to find them in a large metropolis like New York City.

But now all that has changed. Thanks to databases like the one assembled by Ancestry.com, a researcher can find a particular individual in just seconds and then track them across the decades. Federal census records offer little more than a snapshot of American life, and they're compiled just every ten years. But they provide an astonishing amount of information. The census of 1900, for example, not only lets you see where your subject lived, but who lived in their household, when and where they were born, what language they spoke at home, where their parents came from, whether they were literate, and what they did for a living. You can see who lived next door and who lived down the street and imaginatively reconstruct the neighborhood. All of this was possible with microfilm, but it took much, much longer.

With digitized data that you can search in different ways, however, you can also find information few historians would ever have had the patience to assemble before. You can, for example, figure out in a flash how many Chinese-born women lived in New York in 1910 or how many African Americans born in Georgia during slavery times had migrated to Manhattan by 1880.

Since discovering the power and possibilities of the digitized census records, I have been pondering what the creation of such enormous digitized archives of primary source documents means for the practice of history. If we can find information more quickly should we pose more complex questions? Tackle larger subjects? Spend more time polishing our prose?

I have always prided myself on being an "archive rat." I love plowing through documents and images in rare book libraries. I treasure the chance discovery and the unexpected juxtaposition. My last book - about photography in the nineteenth-century West - depended upon archival work in libraries, museums and historical societies throughout the West. But Passing Strange could not have been written without access to online resources. It's a brave new world out there for historians.

Martha A. Sandweiss, Passing Strange, The Penguin Press, Penguin Books