Truman by David McCullough:
To help guarantee a Dewey victory, J. Edgar Hoover was secretly supplying him with all the information the FBI could provide. Dewey and Hoover were old friends and got along well. Hoover had put the resources of the bureau at Dewey’s disposal months before, in the expectation that when Dewey became President he would name Hoover as his Attorney General. “The FBI helped Dewey during the campaign itself by giving him everything we had that could hurt Truman, though there wasn’t much,” remember an assistant to Hoover, William C. Sullivan, who was one of those assigned to cull the files.