By far the best introduction to string theory is: A First Course
in String Theory, by Barton Zwiebach. I recommend that you buy
this if you are at all curious about string theory. This book is
designed for undergraduates with a solid physics grounding in both
third-year quantum mechanics and electromagnetism, and hence should
be accessible to anyone in our graduate programme.
The gold-standard texts for practitioners in the field of string
theory are String Theory (two volumes) by Polchinski and
Superstring Theory (two volumes, older) by Green, Schwarz and
Witten. Serious string theory students should buy all four of these.
All of these texts are published by Cambridge University Press, and
will be on reserve in the UofT Library.
Other advanced notes on string theory & related topics
For lecture notes on black holes and black branes in string
theory, including a lot about entropy, see my TASI-99 notes (PDF, 0.582MB)
.
Michael Peskin has a set of elementary lectures on Conformal Field Theory
(CFT) and string
perturbation theory in his TASI-86
lectures (PDF, 10.3MB)