# Reference for Elliptic PDE on $\mathbb{R}^d$

Could anyone suggest a textbook, article, or lecture notes that covers elliptic PDE theory (existence, uniqueness, regularity) on all of $\mathbb{R}^d$, as opposed to the Dirichlet or Neumann problem on some bounded subset? I emphasize I am interested in a treatment that concerns the elliptic operators with variable coefficients that precludes a Fourier transform based solution. I found that Krylov's textbook (Lectures on Elliptic and Parabolic Equations in Sobolev Spaces) treats the problem, but I was curious if there were any others.