Wikipedia has a good section on Stress (mechanics). It seems to cover all the main principles involved and has great diagrams:
http://en.wikipedia.org/wiki/Stress_(physics)

Wikipedia also has a section on the stress tensor in relativistic theories–the stress-energy tensor:
http://en.wikipedia.org/wiki/Stress-energy_tensor

There is a lecture on the Stress Tensor and the relativistic Stress-Energy Tensor by Stefan Waner of the Department of Mathematics, Hofstra University, at
http://people.hofstra.edu/Stefan_Waner/diff_geom/Sec12.html

A short note about matrix and index notation for the stress tensor is provided by David Roylance of MIT. It deals with the range convention for index notation, the summation convention for index notation and the subscript comma notation to imply differentiation:
http://web.mit.edu/course/3/3.11/www/modules/index.pdf