Tensor Calculus, Part 2 - Discussion of tetrads, commutators, parallel transport, geodesics, Killing vectors, curvature.
Results are useful for gravitational lensing.
Gravitational Lensing from Hamiltonian Dynamics - Deflection of light in a curved spacetime. Symmetry Transformations, the Hilbert Action, and Gauge Invariance - Fundamental principles of GR as a gauge theory. Sean Carroll's General Relativity notes - offers a more mathematical approach.