Group theory is one of the most important tools in quantum field theory. In Quantum Field Theory Demystified, available on Amazon.com, it is discussed in two chapters. I have posted one of those chapters here.
I'm a physicist that happens to own 3 horses and 5 dogs. I do horse training in my spare time and follow expert horse trainer Eric Bravo around when I get a chance, sometimes with a video camera. Email me at firstname.lastname@example.org with questions.