The Venona documents and other evidence later showed that "At least 349 citizens, immigrants, and permanent residents of the United States had a covert relationship with Soviet intelligence agencies, among them Harry White (assistant secretary of the treasury in FDR's administration and the Communists' highest-ranking asset) and State Department official Alger Hiss."

A good book on McCarthy himself is Blacklisted by History: The Untold Story of Senator Joe McCarthy and His Fight Against America's Enemies by M. Stanton Evans. I found it a tad over-the-top in its defense of McCarthy, but mostly good work. TL;DR: McCarthy was mostly right, but went too far (e.g. accusing General George Marshall).

Here's some interesting background on Welch's famous "Have you no decency?" speech. Welch was defending an aide of his who McCarthy had accused of being a communist, but two weeks before, Welch had fired that same aide for... being a communist. So the speech was just faux outrage and theater.