Facing the sturdy red sandstone of Trinity Church across Copley Square, the Boston Public Library hides a staggering collection of Art Nouveau and early 20th-century art behind a serene granite exterior. The credits read like a who’s who of the movers and shakers in the arts world at the turn of the last century: McKim, Mead & White designed the building, Daniel Chester French sculpted the bronze doors, Augustus Saint-Gaudens did the carvings on the façade. Climb the grand staircase inside, between a pair of soulful stone lions, to face the monumental frescoes in the upper gallery eye-to-eye and see the sumptuously-decorated Abbey Room, with the a mural surrounding its upper wall, “The Quest of the Holy Grail” by the American artist Edwin Austin Abbey. If you think these romantic themes, flowing lines and deep colors are pure early century, wait until you see the knock-out stylized murals in the gallery above. Well worth the climb up a seemingly endless marble staircase is John Singer Sargent’s powerful series of murals, “Triumph of Religion” covering the 84-foot-long walls. Learn more about the building’s art and see other masterpieces on a one-hour free tour, offered every day but Wednesday . Bring a sack lunch and join local office workers in the gardens that fill the atrium, or stop by Novel for a lunch buffet of salads, soups, sandwiches and entrees, plus afternoon tea, on weekdays.