"He also built libraries. And he set up in the forum an enormous column..." Cassius Dio, Roman History (LXVIII.16.3)
After Alexandria and Pergamum, the Bibliotheca Ulpia (as it is called by the Historia Augusta) was the most famous library of antiquity and, of all the Roman libraries, the only one to have survived at least until the mid-fifth century AD, when it is mentioned by Sidonius Apollonius, whose statue was installed there (Letters, IX.16.3) by his father-in-law, the emperor Avitus, whom he had honored with a panegyric. Venantius Fortunatus also writes of "Vergil recited in Trajan's forum in the city" (VI.8) about AD 576. Following the tradition of earlier imperial libraries, the Greek and Latin collections were housed separately. In the Ulpian library, they faced one another across a a small colonnaded courtyard that enclosed the Column of Trajan.
In this computer reconstruction of the west (Greek) library by John Burge (in consultation with James Packer and Kevin Sarring), one looks through bronze screens into the portico, where the base of the Column of Trajan can be seen. Facing east, as Vitruvius recommends for libraries (VI.4), the screens restricted access when the library was not in use and, with the high vaulted ceiling, took advantage of the morning light.
Portions of the floor and the podium of one of the walls survive, which, together with fragments in storerooms beneath the Via dei Fori Imperiali, allow reconstruction of the library interior. By multiplying the diameter of columns and adding the height of the entablature (fragments of the egg-and-dental cornice survive, as well), Packer and his colleagues were able to calculate that the upper tier of the library was three-quarters as tall as the lower one, the ratio prescribed by Vitruvius for the columns of basilicas (V.1).
As can be seen in the reconstruction, the walls were broken into bays by fluted Corinthian columns set opposite pilasters that framed niches which held the cabinets for the books. Three steps between the columns allowed access to a walkway in front of the bookcases, themselves. At the other end of the hall were recesses for a statue on each level, presumably of Trajan and possibly of Minerva.
The sumptuous room was paved in gray Egyptian granite separated by strips of golden purple-veined giallo antico from Numibia in North Africa. The walls were covered with a veneer of pavonazzetto marble (pavo, peacock, because of the contrasting purple veins) from Phrygia in western Asia Minor. The columns, too, were of the same variegated marble, whereas the niches holding the scrolls were framed in white marble, as were the capitals and bases of the columns.
The scrolls, themselves, were stored in armaria recessed in the wall, protected in wooden bookcases from the damp. There were seven niches on each side of both walls and four across the back, two on each level, flanking the statues. It has been estimated that these thirty-six cabinets held approximately ten thousand scrolls, with a like number in the east (Latin) library. In addition, there were archival materials, such as praetorian edicts (Aulus Gellius, XI.17.1) and senatorial decrees (Augustan History, Tacitus, VIII.1), as well as Caesar's autobiography and Trajan's commentaries on the Dacian Wars, of which now only a few words survive. With desks (plutei) and the books, themselves, on shelves out of sight, the space was designed for reading. But, although aesthetically pleasing, there also was the risk of a lack of space as the collection grew. Indeed, the Historia Augusta (Aurelian, 1.7; Probus, 2.1) indicates that the linen books were transferred to the Baths of Diocletian sometime in the fourth century, a time, laments Ammianus (XIV, 6.18), when "the libraries are like tombs, permanently shut." Although these linen books likely are fictitious, as are the "books of ivory" mentioned in the Life of Tacitus (VIII.1), the author does relate that he was able to procure books in Greek from the Ulpian library and "laid my hands on all that I needed" (Aurelian, I.9).
Isidore of Seville notes that architects used green Carystean marble (cipollino verde) to panel libraries because the color was thought to refresh the eyes (Etymologies, VI.11.2).
Reference: The Forum of Trajan in Rome: A Study of the Monuments in Brief (2001) by James E. Packer; Libraries in the Ancient World (2001) by Lionel Casson; Scriptores Historiae Augustae (1921-) translated by David Magie (Loeb Classical Library); Ammianus Marcellinus: The Later Roman Empire (1986) translated by Walter Hamilton (Penguin Classics); Venantius Fortunatus: Personal and Political Poems (1995) translated by Judith George.