Could anyone recommend some good books on string?

[Dale12]

I have some material:
1.Joseph Polchinski <String theory>/An Introduction to the Bosonic String.
2.Richard J.Szabo <BUSSTEPP lectures on string theory>
hep-th/0207142
3.Elias Kiritsis <Introduction to superstringtheory>hep-th/9709062
and I don't know which is better,and it is difficult to read them, could some one give some advice?thank you very much!

 

[selfAdjoint]

I found myself that it's horrible to try to learn string theory if you don't have a pretty strong background in quantum field theory. Students of any of the books you cite are expected to have seen things like the operator product expansion before.

I don't know what your level is, but if you are comfortable with the mathematical operations but have problems with the physics I could recommend Hatfield's Quantum Field Theory of Point Particles and Strings. It's like a quicky intro to field theory as a preparation for string theory. It's not a standalone string theory course, but serves as a prelimnary to books like Polchinski.

If on the other hand you are having trouble with the math, then you have to go way back and start with advanced calculus.

I don't know of any book, or online site, that has more detail than The Elegant Universe, but doesn't require serious background and hard work.

 

[Dale12]

I am a graduated student but not so good at learning. we have learned Quantum Fields Theory (I) and are learning (II)(especially path integral).but to learn string, I still feel difficult to go on. some name seems so strange, such as light coordination.Planck length,Planck mass,p-brane,reparametrization invariance,and I still couldn't understand what is the meaning of weyl invariance.and etc.
And when reading book,I found my knowledge on group theory is not enough, but to read all of them seems to be not possible.the same as differential geometry.
thanks anyway!