"Probably the best place to start for f(R), and general curvature scalar-functional field theories. Wonder why String Theorists give "strange combinations" of R_ijkl in actions as Gravity predictions?"
"Antiquated, but very much ahead of its time. MKSA throughout; with the full, tensorial derivation of the Lorentz force law, among other advanced topics in applications of Maxwellian Electro-Magnetism."
"A very old, simple book. Feynman himself used it to learn Quantum Physics [and attempt a crude Gravity quantisation from the Schrödinger operator rules whilst at MIT; oddly similar to Wheeler-DeWitt!]"