I've been in Dubna last year for about a month.I had alist of about 7 books on superstrings,and, the only one in Enlglish i found at the "Bogoliubov Laboratory of Theoretical Physics" was the one written by Lars Brink and Marc Henneaux "Principles of string theory".Havind just read Dirac's book"Principles of QM",i thought "It can't be that difficult".I opened the book and never got beyond the first page where lied only one formula:the Born-Infeld action.Obviously,having no idea abot that thing,i found no point in reading the next pages.
Satisfied with my answer?
The born-infeld action governs the dynamics of higher dimensional objects called D-branes. Over the last decade they've helped reveal a great deal about nonperturbative aspects of string theory and in particular that what appeared to be many different theories are actually different backgrounds of a unique theory called M-theory. But to understand D-branes one must first learn the basics of string theory since D-branes first appear, in connection with a symmetry known as T-duality, as surfaces on which open strings end.