[en] We report on a precise study of the x and Q² dependence of the proton deuteron and neutron structure functions. In particular R = σ_L/σ_T, F_2p, F_2d, and the ratio F_2d/F_2p were extracted in a combined analysis of SLAC deep inelastic electron scattering experiments. Data from eight experiments were radiatively corrected with a new improved radiative correction formalism, and all experiments were normalized to the precise data of SLAC E140, thus yielding a single coherent data set. We find that R_p = R_d as expected from QCD. The results for R are somewhat larger than the prediction of QCD with target mass effects, thus indicating the existence of additional dynamic higher twist effects. Using both SLAC and CERN data, we obtained a parametrization of R in the range 0.5< Q²<200 GeV/c² and 0.1< x<0.86. This fit in conjunction with the normalized hydrogen and deuterium cross sections was used to extract precise values of F₂. These data provide a constraint at low Q² for high Q² muon and neutrino experiments. A study of the Q² dependence of the ratio of deuteron and proton structure functions shows a slope which is consistent with QCD and explains the difference in this ratio between SLAC and CERN experiments. (data)