I just finished Homeland : a novel by Barbara Hambly. It's letters between two women - one in Maine and one in Tennessee/Georgia (she moves a lot) - during the US Civil War.
This is an excellent book; I totally recommend it.
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I'm about to start Sex Wars by Marge Piercy. Book's plot: set in post–Civil War New York stars a true-life cast of characters that includes Victoria Woodhull, the spiritualist turned first woman to run for the U.S. presidency; passionate suffragette Elizabeth Cady Stanton; the aged Cornelius Vanderbilt, who sits atop a $100-million fortune as he tries to make contact with his dead son; and Anthony Comstock, a crusading moralist who dedicates his life to outlawing pornography and "obscene objects made of rubber." As they each vie for different kinds of sex-based power, the consequences of their actions echo from the halls of Congress to Manhattan's back alleys.