I'm studying the book "string theory" by Becker, Becker and scwarz. Unfortunately, I don't understand really well the materials in Chapter 3 "conformal field theory and string interactions". I don't understand OPE (operator product expansion) concept and that why the OPE of a scalar field, like $X^{\mu}$, has the form $X^{\mu}(z)X^{\nu}=-\frac{1}{4}\eta^{\mu\nu}\ln (z-w)+\cdots $ and why we sometimes compute $\langle \partial X^{\mu}(z)\partial X^{\nu}(w)\rangle $ the corellation function. 

I would really appreciate if someone could introduce me an easier book to understand them. Thank in advance.