Who were the Shudras?

How does one expose the hypocrisy of an orthodoxy? By studying their sources of 'truth' and invalidate them. That's the approach taken by BR Ambedkar in his book "Who were the Shudras?". He quotes the Vedas and other scriptures to demonstrate tampering and selective reading as a basis to prop up the caste system. In addition, he cites scientific studies (one theory regarding the head size which has since been debunked) to prove the origin of the Shudras and put the Aryan invasion theory to doubt. Quite a hard-hitting book!