I'm studying the book "String Theory" by Becker, Becker and Schwarz. 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 correlation function.

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