[Samuel W. McCall wrote a fantastic biography in 1899.] (https://archive.org/details/thaddeusstevens00inmcca) Since the book was first published before 1923, it's available in its entirety for free, and the link I just gave you will take you to the book. If you want a paper copy, however, you can get one for around ten bucks. It is a good book to physically have if you're planning on doing any research (you can write notes on it and stuff). If it's just to read, on the other hand, I'd recommend just giving the link a go.

More recently, in the 60s, [Fawn Brodie wrote what is considered to be perhaps the definitive biography on Stevens--- Thaddeus Stevens: Scourge of the South.] (http://www.amazon.com/Thaddeus-Stevens-Fawn-M-Brodie/dp/0393003310/ref=sr_1_1?s=books&ie=UTF8&qid=1459785820&sr=1-1&keywords=thaddeus+stevens) The book straddles the line between "too new to be available for free" and "too old to be at most public libraries," but if you are near a major public or university library system, you should be able to get a copy there. Still, you can get a copy for around twenty bucks, and it is well worth the price.

Lastly, Stevens is features pretty heavily in Doris Kearns Goodwin's Pulitzer Prize-winning [Team of Rivals: The Political Genius of Abraham Lincoln] (http://www.amazon.com/Team-Rivals-Political-Abraham-Lincoln/dp/0743270754). This book will be ubiquitous in any decent public library, and it was such a best-seller when it was published in 2005 that you can now get used copies for dirt cheap--- 72 cents plus shipping on Amazon right now.

That's all I got. I hope I could help, and I'll be glad to answer any questions you might have. Happy reading!