What mix of emotions bubbles up when you see a photograph of a lynching? Do you feel sad or mad, curious or confused, numb or nervous? Where do your eyes go? Do they behold the disfigured black body, mangled and mistreated? Do they look to the white children in the crowd, or the smiling young woman who appears ready for a date, or the stoic white men who might have tightened the noose? Do you look at the physical surroundings? Perhaps the trees or the signposts catch your eyes. Or do you look away? What we look at and how we feel about it may speak volumes to our attitudes toward race in general. At times in the 20th and 21st centuries, Americans have been fascinated and repulsed by lynching photographs, often simultaneously. From the Civil War to World War II, at least 5,000 black Americans, usually men, were brutally murdered by white mobs. The victim was often beaten, tortured, burned alive, and dismembered. Sometimes he had his genitals mutilated; sometimes he was decapitated. Sometimes the white crowds took souvenirs, such as knuckles, locks of hair, or blood dripped onto paper. Usually no one was prosecuted.

Time and again, these wicked acts were photographed. The photos were then purchased (or given away) as souvenirs. Postcards of the events were reproduced, sold, mailed, and collected. “Lynching Photographs” (University of California Press, 101 pages, $50), by Dora Apel and Shawn Michelle Smith, is a book about how and why these events were photographed. Ms. Smith focuses on Lawrence Beitler’s photograph of the lynching of Thomas Shipp and Abram Smith in Marion, Ind., in 1930. This is one of the most well known lynching photographs: Shipp and Smith are in the center, suspended from trees. In the foreground, a white man points at the bodies. A white couple stands close together; they hold hands and smile, in an incongruous expression of warm intimacy. Some older whites simply stare at the black bodies. Ms. Smith established herself as an expert on visual culture with her book “Photography on the Color Line” (2004), on the use of images by activist and intellectual W. E. B. DuBois. In “Lynching Photographs,” she traces how the Marion photograph was used for a startling variety of purposes throughout the 20th century. African-American newspapers in the 1930s, including the Chicago Defender and the Crisis, published the photo as evidence of the evils of American racism; rap group Public Enemy used it in 1992 to condemn the so-called “lynching” of boxer Mike Tyson, who had been accused of rape; Justice for All, an anti-abortion organization, has used it to link the evil of lynching with the practice of abortion, and several senators claimed that it played a role in their 2005 Anti-Lynching Apology Resolution. In fact, during the Senate’s televised apology, the photo of Shipp and Smith sat on the speaker’s platform.

Lynching photographs constitute “neither univocal nor fixed evidence,” Ms. Smith concludes, but offer meaning shaped by context and circulation. Ms. Apel, who has written provocative books on the imagery of lynching and of the Holocaust, follows with an intriguing essay on how lynching photographs, once icons of white racism, came to be used against it. She wonders how images that were first produced to evoke white pride and entitlement now elicit outrage, guilt, and shame: Were lynching photographers the unwitting architects of lynching’s demise? Or is the changing meaning of those photographs merely the result of our attitudes toward race and the law? Ms. Apel believes that photographs played an active role in the end of lynching, and focuses upon five defining moments in this shift. Most noteworthy was the murder of Emmett Till in 1955, and the “spectacle funeral” staged by his mother, where white and black Americans viewed his body not hung from a tree, but respectfully in a coffin. In this case, Till’s mother, and not a white photographer, controlled the representation of her son, reclaiming his memory from the lynchers and photographers. Ms. Apel also connects the images of lynching to those of Rodney King’s beating, the murder of James Byrd Jr., in 1998, and the photos of the tortured prisoners at Abu Ghraib.

This is not a book for the weak of stomach, heart, or knee. While “Lynching Photographs” contains but a few photographs, they are hard to look at. Sadly, overlooking W. E. B. DuBois, Langston Hughes, Countee Cullen, and Georgia Douglas, who each reinvented the figure of the lynched man as a martyr through art, Ms. Smith and Ms. Apel provide no means to address the fear and sadness raised by these images. Whatever their context, however, these photographs themselves inspire terror.

Mr. Blum is a professor of history at San Diego State University and the author of the award-winning “Reforging the White Republic: Race, Religion, and American Nationalism, 1865-1898” (2005), and “W. E. B. Du Bois, American Prophet” (2007).