Isidore of Seville, Etymologiae 6.5-6
V. On the one who first brought books to Rome:
Aemilius Paulus was the first to bring a supply of books to Rome, following the defeat of Perseus king of Macedon. Then, Lucullus brought some from his Pontic plundering. Later than these guys, Caesar gave to Marcus Varro the task of building the biggest possible library. Pollio, however, was the first to make public libraries in Rome (both Greek and Latin ones), and he added busts of the authors in the atrium, which he had made rather magnificent with some of the money he had obtained by spoil.
VI. Those who founded libraries among us:
Among us, Pamphilus the Martyr, whose life was written by Eusebius of Caesarea, strove to equal Pisistratus in his zeal for a sacred library. For he had almost three hundred thousand books in his library. Jerome, too, and Gennadius, searching the whole world for ecclesiastical authors, followed them in order, and comprehended their studies in one little index of a volume.
V. DE EO QVI PRIMVM ROMAM LIBROS ADVEXIT.  Romae primus librorum copiam advexit Aemilius Paulus, Perse Macedonum rege devicto; deinde Lucullus e Pontica praeda. Post hos Caesar dedit Marco Varroni negotium quam maximae bibliothecae construendae.  Primum autem Romae bibliothecas publicavit Pollio, Graecas simul atque Latinas, additis auctorum imaginibus in atrio, quod de manubiis magnificentissimum instruxerat.
VI. QVI APVD NOS BIBLIOTHECAS INSTITVERVNT.  Apud nos quoque Pamphilus martyr, cuius vitam Eusebius Caesariensis conscripsit, Pisistratum in sacrae bibliothecae studio primus adaequare contendit. Hic enim in bibliotheca sua prope triginta voluminum milia habuit.  Hieronymus quoque atque Gennadius ecclesiasticos scriptores toto orbe quaerentes ordine persecuti sunt, eorumque studia in uno voluminis indiculo conprehenderunt.