Credit...Christopher Anderson/Magnum Photos When Heather McGhee was a 25-year-old staffer at Demos, the progressive think tank she would eventually lead, she went to Congress to present findings on shocking increases in individual and family debt. “Few politicians in Washington knew what it was like to have bill collectors incessantly ringing their phones about balances that kept growing every month,” McGhee writes in her new book, “The Sum of Us: What Racism Costs Everyone and How We Can Prosper Together.” Demos’s explanatory attempts failed. When Congress finally took action in 2005, it made the problem worse, passing a bankruptcy bill that made escaping unsustainable debt harder than ever. For McGhee, the disaster was an education in the limits of research, which is often no match for the brute power of big money. But as she was walking down the hallway of the Russell Senate Office Building, she learned something else.