L. D. Faddeev 
Gauge Fields [PDF ebook] 
An Introduction To Quantum Theory, Second Edition

Gauge field theory describes the physics of elementary particles adequately at moderate energies. Besides, the methods applied in the field theory of relativistic strings represent a direct generalization of the methods of gauge field theory, to which this book is devoted. For this reason the author considers a new edition of it to be useful, both for direct applications of the already developed gauge theory and for search of new ways.