QFT

"The career of a young theoretical physicist consists of treating the harmonic oscillator in ever-increasing levels of abstraction."

      -- Sidney Coleman. 

It is totally worth giving a look at the following website for doing some independent study on QFT. 

Lectures on Quantum Field Theory- David Tong

Note: Let me warn you here, that the subject matter discussed here is highly technical and that undergraduate physics is strictly required to follow up the course.

I hope this helps.