Relativistic extension of quantum mechanics will extend naturally into quantum field theory and on a beginner level, my suggestions would be to follow the book by Lewis H. Ryder and Mark Srednicki. A more theoretical approach can be found in the book named "Quantum Field Theory in a nutshell" by Anthony Zee. QFT books of Peskin and Schroeder, and Weinberg Volume 1 are the ultimate books to go to in the end.