I would like some references concerning the following subject.
Suppose that $\Omega$ is a bounded subset in $\mathbb{R}^n$ with smooth boundary and consider the following PDE there stated $$L(f)(x) = g(x),~ x\in \Omega,~~f(y) = h(y),~y\in \partial \Omega,$$ where $L : W^{2,p}(\overline \Omega) \to L^p(\overline \Omega)$ is an elliptic operator in $\Omega$ but such that its principal symbol vanishes for points in $\partial \Omega$.
Which are some classical references concerning this kind of problem, such as, existence of solutions and eventual regularity theory?