We prove explicit rationality-results for Asai-L-functions, L S (s, Π ′ , As ± ), and Rankin-Selberg L-functions, L S (s, Π × Π ′ ), over arbitrary CM-fields F , relating critical values to explicit powers of (2πi). Besides determining the contribution of archimedean zeta-integrals to our formulas as concrete powers of (2πi), it is one of the crucial advantages of our refined approach, that it applies to very general non-cuspidal isobaric automorphic representations Π ′ of GLn(AF ). As a major application, this enables us to establish a certain algebraic version of the Gan-Gross-Prasad conjecture, as refined by N. Harris, for totally definite unitary groups: This generalizes a deep result of Zhang and complements totally recent progress of Beuzard-Plessis. As another application we obtain a generalization of an important result of Harder-Raghuram on quotients of consecutive critical values, proved by them for totally real fields, and achieved here for arbitrary CM-fields F and pairs (Π, Π ′ ) of relative rank one.