Pro tem : presidents pro tempore of the United States Senate since 1789 / prepared by the Senate Historical Office under the direction of Nancy Erickson, Secretary of the Senate ; with a preface by Senator Robert C. Byrd, President pro tempore.
Foreword -- Preface -- The formative years, 1789-1860 -- A question of succession, 1861-1889 -- Fathers of the Senate, 1890-1946 -- The modern era, 1947-present.