Michio Kaku 
Introduction to Superstrings and M-Theory [PDF ebook] 

Superstrings — provocative, controversial, possibly untestable, but unarguably one of the most interesting and active areas of research in current physics. Called by some, "the theory of everything", superstrings may solve a problem which has eluded physicists for the past 50 years — the final unification of the two great theories of the twentieth century, general relativity and quantum field theory. Now, here is a course-tested comprehensive introductory graduate text on superstrings which stresses the most current areas of interest, not covered in other presentation, including: — string field theory — multi loops — Teichmueller spaces — conformal field theory — four-dimensional strings The book begins with a simple discussion of point particle theory, and uses the Feynman path integral technique to unify the presentation of superstrings. Prerequisites are an aquaintance with quantum mechanics and relativity. This second edition has been revised and updated throughout.