I would definitely suggest restricting yourself to recent historical works, as Thaddeus Stevens was Public Enemy Number One for the Dunning School, so got cast as the Richard III of the Radical Republicans.
Here’s some recent books I can recommend:
- The Unknown Architects of Civil Rights, Barry Goldenburg, 2011.
- Thaddeus Stevens: Nineteenth Century Egalitarian, Hans Trefousse, 2000.