In Morgan's day, visitors to the library passed through a pair of monumental bronze doors into a rotunda replete with opulent detail: variegated marble columns, an ornately patterned floor, and fine mosaic panels that line the curved walls. The ceiling paintings, by American artist H. Siddons Mowbray (1858–1928), depict three of the major literary epochs represented in Pierpont Morgan's collections—the ancient world, the Middle Ages, and the Renaissance. The apse, decorated with blue and white stucco reliefs of classical and mythological subjects, is based upon Raphael's design for the Villa Madama in Rome. Mowbray modeled the reliefs on site to ensure they would be lighted effectively by the central oculus, or skylight. Over the door opposite the library's grand entrance, a ceramic relief by the Renaissance sculptor Luca della Robbia (1400–1482) depicts the madonna and child framed by the Latin motto Soli deo honor et gloria (Glory and honor to God alone).
On view in the Rotunda are highlights from the Morgan's distinguished collection of rare printed and manuscript Americana. As Pierpont Morgan strove to build an American library that would rival the great collections of Europe, he did not overlook the history and literature of his own country. He acquired important letters of George Washington and Abraham Lincoln as well as collections documenting the early years of the republic. In 1909 he purchased some of the finest surviving literary manuscripts of nineteenth-century America, including the journals of Henry David Thoreau and Nathaniel Hawthorne and works by Edgar Allan Poe and Ralph Waldo Emerson. In recent years, the Morgan has continued to augment its American holdings, acquiring the Carter Burden Collection of American Literature and the archives of the literary periodical The Paris Review. Featured selections are changed every few months.