Here are some online resources on Field Theory related issues:
- Hagen Kleinert‘s Books.
- Warren Siegel‘s Fields.
- Warren Siegel‘s Introduction to string field theory.
- Srednicki‘s Quantum Field Theory.
Though I’m hardly a specialist in this (yet!) I can see that all of these books are great and would be a great help for anyone wanting to enter this exciting field of research.
Kleinert also has this page were his lecture notes are available. My recommendation is for you to download his Particle and Quantum Fields files.