Elliptic PDE on the Whole Space

Question

Can anyone suggest a reference for elliptic PDE on the all of $\mathbb{R}^d$, as opposed to some bounded domain $\Omega$, covering the standard topics of existence, uniqueness, and regularity. I specifically mean elliptic operators with variable coefficients, so these problems cannot be automatically treated by the Fourier transform. The only reference I've found is Krylov's book. Everything else (Gilbarg & Trudinger, Evans), seem to be limit themselves to bounded subsets with sufficiently regular boundary.