Answer:
The other half referred to by Jacob Riis in the title of his book is the poor people that live in the slums of American cities.
Step-by-step explanation:
Jacob Riis used the newly born photography to document and to show the world how were the living conditions of poor people in cities who lived in tenements and homeless shelters.
Riis himself was a Danish migrant and had lived like this when he arrived in the US. He knew how hard that life was so he wanted to show it to the other parts of society that had no idea about it: upper and middle classes.