String Theory: An Introduction to the Bosonic String: Volume 1 (Cambridge Monographs on Mathematical Physics) Hardcover – 22 October 1998

This is the standard text to learn string theory. This volume covers the bosonic string, its quantization and spectrum, tree level and one loop amplitudes, D-branes, and some comments about higher level amplitudes. The book is quite well-written. Everything is presented with great care, but not always with great detail. Many calculations are skipped, or just barely given. It also assumes that you are very comfortable with the language and techniques of non-abelian gauge theories. As Ron Maimon says on physics.stackexchange, you need to have read at least the first volume of Weinberg's QFT book. I have not (yet), and I believe I did not understand more than half (by a conservative estimate) of the 'physical/philosophical' comments made in the book on the general nature of string theory. I hope to get back to it after reading Weinberg.On the whole though, this book will make you familiar with many great things. The price to pay is the very large background required, and the huge effort to really get through the book.