String Theory Books

String theory is a difficult subject that is still evolving. As a field of study becomes more familiar and better understood, the books that are published about it start becoming addressed to a wider audience (professional → student → layperson). There are plenty of popular books on the subject, but IMHO the more relevant indication of a field's conceptual progress is the availability of undergraduate level texts (practitioner → graduate student → undergraduate student). String theory is starting to approach the advanced undergraduate level, as evidenced by these books in reverse chronological order:

• String Theory Demystified
  (2008), undergraduate/graduate
  by David McMahon
• String Theory in a Nutshell
  (2007), graduate, errata
  by Elias Kiritsis
  
• String Theory and M-Theory: A Modern Introduction
  (2007), graduate
  by Katrin Becker, Melanie Becker, & John H. Schwarz
  
• Supersymmetry and String Theory: Beyond the Standard Model
  (2007), graduate/practitioner, errata
  by Michael Dine
  
• An Introduction to String Theory and D-Brane Dynamics
  (2004), graduate/practitioner
  by Richard J. Szabo
• A First Course in String Theory
  (2004, 2nd Ed. in Feb 2009), undergraduate/graduate
  by Barton Zwiebach
  
• D-Branes
  (hardcover 2003, paperback 2006), practitioner, errata
  by Clifford V. Johnson
  
• String Theory, Volume I: An Introduction to the Bosonic String
  String Theory, Volume II: Superstring Theory and Beyond
  (hardcover 1998, paperback 2005), graduate, errata for both volumes
  by Joseph Polchinski
  
• Quantum Field Theory of Point Particles and Strings
  (1998), graduate
  by Brian Hatfield
  
• Supersymmetric Gauge Field Theory and String Theory
  (1994), graduate/practitioner, errata
  by D. Bailin & A. Love
  
• Introduction to Superstrings and M-Theory
  (1988, 2nd Ed. 1999), practitioner
  by Michio Kaku
  
• Superstring Theory: Volume I, Introduction
  Superstring Theory: Volume 2, Loop Amplitudes, Anomalies and Phenomenology
  (hardcover 1987, paperback 1988), graduate/practitioner
  by Michael B. Green, John H. Schwarz, & Edward Witten