By Dr. Ashoke Sen:

“ For the understanding 'String Theory', following are the pre-requisitions:

1. Classical Mechanics by Goldstein
2. Classical Electrodynamics by Jackson
3. Quantum Electrodynamics by Greiner
4. Quantum Electrodynamics by R. Feynman
5. Advanced Quantum Mechanics by Sakurai
6. Quantum Gravity by Carlo Rovelli
7. Quantum Field Theory by Mandl and Shaw
8. Advanced General Theory of Relativity by John Stewart

And before starting these books, you have to read a few more books for Basic Build up. Those are :
1. Classical Mechanics by Taylor
2. Waves and Oscillation by Walter Fox Smith
3. Vibrations and Waves by AP French
4. Introduction to classical electrodynamics by David J. Griffiths
5. Quantum Mechanics by Shankar
6. Modern Quantum Mechanics by Sakurai

& before that One needs to learn the necessary mathematics :

1. Basic Differentiation and integration
2. Ordinary and Partial Differential Equations
3. Linear Algebra
4. Complex Analysis
5. Vector Analysis
6. Probability and Statistics
7. Group Theory and Representation Theory
8. Calculus of variations / Multivariate Calculus
9. Series and Special Functions
10. Differential Geometry
11. Tensor Analysis
12. Clifford Algebra
13. Lie Algebra ( will be covered in advanced group theory)
14. Riemann and Non Euclidean Geometry
15. Banach Algebra and many more

After these finishes, I think that the string theory will be somewhat understandable. ”

• Ashoke Sen's Home Page
• Asoke Sen on ResearchGate
• Asoke Sen on Google Scholar