I found, with no reference, the following theorem which is called Besicovitch derivation theorem. Do you know any article/book where I can find this powerful result?
Let $\Omega\subset \mathbb R^n$ be an open set, $\mu,\nu$ be positive measures on $\Omega$. Then, for $\mu-e.a.\ x$ in the support of $\mu,$ $$\exists \ \ \lim_{\rho \to 0} \frac{\nu (B_\rho (x))}{\mu( B_\rho(x))}=:f(x)$$ moreover the Radon-Nykodim decomposition of $\nu$ is given by $$\nu=f\mu+\nu^s$$ Finally, defining $$E:=\biggl (\Omega-supp(\mu)\biggr ) \bigcup \{x: f(x)=\infty\}$$ we have $$\mu(E)=0 \qquad \nu^s(\Omega - E)=0$$