The building includes lavish decorations, a children's room (the first in the nation), and a central courtyard surrounded by an arcaded gallery in the manner of a Renaissance cloister.

McKim drew explicitly on the Bibliothèque Sainte-Geneviève in Paris (designed by Henri Labrouste, built 1845 to 1851) for the general arrangement of the facade that fronts on Copley Square, but his detailing of that facade's arcaded windows owes a clear debt to the side elevations of Leon Battista Alberti's Tempio Malatestiano in Rimini.

[3] Significantly, the ceramic tile patterns are exposed to view on the interior, while all previous projects by Guastavino Sr. covered the vaults with plaster.

Within each arch are sets of bronze doors by Daniel Chester French added in 1904, which depict left to right, low-relief allegories of Music and Poetry; Knowledge and Wisdom; and Truth and Romance.

On the south is inscribed: "MDCCCLII • FOUNDED THROUGH THE MUNIFICENCE AND PUBLIC SPIRIT OF CITIZENS"; on the east: "THE PUBLIC LIBRARY OF THE CITY OF BOSTON • BUILT BY THE PEOPLE AND DEDICATED TO THE ADVANCEMENT OF LEARNING • A.D. MDCCCLXXXVIII"; and on the north: "THE COMMONWEALTH REQUIRES THE EDUCATION OF THE PEOPLE AS THE SAFEGUARD OF ORDER AND LIBERTY".

Below each second-story arched window on the three façades are inscribed lists of the names of great historical writers, artists, scientists, philosophers, and statesmen.

The east side has a rhythmic series of arched windows with light buffered by wide overhanging hood on the exterior.

It possesses wide-ranging and important holdings, including medieval manuscripts and incunabula, early editions of William Shakespeare (among which are a number of Shakespeare quartos and the First Folio), the George Ticknor collection of Spanish literature, a major collection of Daniel Defoe, records of colonial Boston, the 3,800 volume personal library of John Adams, the mathematical and astronomical library of Nathaniel Bowditch, important manuscript archives on abolitionism, including the papers of William Lloyd Garrison, and a major collection of materials on the Sacco and Vanzetti case.

The library has a special strength in music, and holds the archives of the Handel and Haydn Society, scores from the estate of Serge Koussevitzky, and the papers of the important American composer Walter Piston.