Strings and their theory
The course will follow but not necessarily be identical with these lecture notes (in German, english version to follow at some point).
Good introduction.
One of the standard textbooks on advanced aspects of string theory.
Until some time ago the standard textbook, explains well and in sufficient detail the state of the art before 1987, but does not contain more recent developments.
Very good popular science exposition of string theory, also recommended for physics students.