Lectures of Prof. Ashok Sen on Thermal Field Theory https://www.youtube.com/playlist?list=PLB5CVZSR7aDyWsnElpjiU8ahQ0W8uDdCe might be valuable, which are based on the book of M. Laine and A. Vuorinen, Basics of Thermal Field Theory, vol. 925 of Lecture Notes in Physics, 2016.
