I'm in my first course of PDE and I need to investigate the proof of Wiener's Criterion for Laplace Equation which says, if $\Omega \subset \mathbb{R}^n$$(n>2)$ is a bounded domain and $\partial \Omega$ is smooth, then
A point $x_0 \in \partial \Omega$ is regular if and only if the series $$\sum_{j=0}^{\infty} \frac{C_j}{\lambda^{j(n-2)}} $$ diverges. Where $\lambda$ is a fix point of $(0,1)$ and $$C_j = cap(\{x \notin \Omega : |x-x_0| \leq \lambda^j \})$$
This is the statement of Gilbarg's book (page 28).
Now my problem is that I cannot find a explicit proof of this result, except in the Kellogg's book (Foundations of the Potential Theory), but the book is old and I don't understand because the notation is different and is a scanned version, then my question is if there exists other book with the proof of this?