Why do you think DuBois's The Souls of Black Folk and "The Souls of White Folk" are both important in terms of understanding racial or ethnic inequality?

Both The Souls of Black Folk and "The Souls of White Folk," by W. E. B. DuBois, are important for an understanding of racial or ethnic inequality in the United States. You can understand that problem reading only one of those works, but if you read them both, then you'll understand the problem better.
The first work, The Souls of Black Folk, is Mr. DuBois's explanation of what it's like to be African American. His notion of the dual consciousness or dual perception describes the double life that black people lead in the US, one eye on themselves and one eye on everyone else. This could be true of everyone, but it's got far more serious consequences for people who aren't white. The book was important, and remains so, because it was the first public, high-profile, and popular explanation of what it was like to be black in America.
The second work, "The Souls of White Folk," is DuBois's essay about what white people have done to black people. It's not an essay about what it's like to be white. Rather, it's about the injustice of the difference between being white and being black. It sets blackness in historical context.
Reading both works while trying to understand racial inequality in the United States gives you a better understanding of the problem than reading just one of them, because you will learn how blackness developed, how whiteness developed, and how the differences between them came about. You'll also learn how those differences came to be part of American culture, and you'll get some insight into why they persist.