String theory for non-physicists
see also Colloquia
The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory
by Brian Greene (1999)
One of the most popular books on the subject. The first part gives a basic overview of strings as well as background material on relativity and quantum mechanics. The second part describes more advanced ideas concerning details of the extra dimensions required by string theory.
The Fabric of the Cosmos: Space, Time and the Texture of the Cosmos
by Brian Greene (2004)
Hyperspace: A Scientific Odyssey Through Parallel Universes, Time Warps and the Tenth Dimension
by Michio Kaku (Oxford University Press, 1994)
This work focuses on the extra dimensions of string theory, including much background material.
Superstrings: A Theory of Everything?
by P. C. W. Davies (Editor) and Julian Brown (Editor) (Cambridge University Press, 1988)
This book contains interviews with various physicists including Green, Gross, Witten, Feynman, Salam and Weinberg who give their views on Superstring Theory.
NOVA: The Elegant Universe
by Brian Greene (PBS, 2003)
This documentary, based on Brian Greene's book of the same name, is an excellent introduction to string theory. It is available split up into short sections on the PBS website for The Elegant Universe; you can also find them joined up into the three parts on Google Video (legally?) if you search for "the elegant universe".
The Official String Theory Web Site
by Patricia Schwarz
This site addresses string theory at many levels and is appropriate for a broad audience. The basic version presents a broad overview without equations. The reader can also learn about the history of the subject and some of the people involved. Physics topics addressed include Basics of Strings, Experiments, Cosmology, and Black Holes.
Beyond String Theory
by Jan Troost
In addition to topics such as accelerators, relativity, string theory, unification, banes, M-theory, the site also includes a number of "Intermezzos" on sociology, nobel prizes, and the like.
by John M. Pierre
has a tutorial and further links on string theory
Desperately Seeking Superstrings
by P. Ginsparg and S. Glashow (physics/9403001, 3 pages)
This provides a popular-level critique of string theory as of 1986. Though now somewhat dated, many of the comments are still relevant to string theory today, providing a sober contrast to most other popular accounts.
String Theory: a theory in search of an experiment
by H. J. Schnitzer (physics/0311047, 7 pages)
A sober discussion of string theory which comments on many modern developments. While written at the elementary level, many references are made to more advanced concepts.