Taylor, Michael Eugene. Partial differential equations I: Basic theory. Vol. 1. Springer, 1996.

Gallot, Sylvestre, Dominique Hulin, and Jacques Lafontaine. Riemannian geometry. Springer, 2004.

Salo, Mikko. The Calderon problem on Riemannian manifolds. Chapter to appear in Inside out, edited by G. Uhlmann.
(http://www.rni.helsinki.fi/~msa/pub/Salo_InsideOut.pdf)