A nonperturbative electron transfer rate theory is developed on the basis of reduced density matrix dynamics, which can be evaluated readily for the Debye solvent model without further approximation. Not only does it recover for reaction rates the celebrated Marcus' inversion and Kramers' turnover behaviors, but the present theory also predicts reaction thermodynamics, such as equilibrium Gibbs free energy and entropy, some interesting solvent-dependent features that are calling for experimental verification. Moreover, a continued fraction Green's function formalism is also constructed, which can be used together with the Dyson equation technique for efficient evaluation of nonperturbative reduced density matrix dynamics.