Complex sentence: During the war, she authored her diary ("The Diary of Miss Emma Holmes", published in 1979 by the Louisiana University Press) which has since become historically significant due to her in-depth accounting of events occurring during the American Civil War.
Split into simpler sentences: During the war she kept a diary which was published as "The Diary of Miss Emma Holmes" in 1979 by the Louisiana University Press. This is historically significant due to her in-depth accounting of events occurring during the American Civil War.