The small Isabella Stewart Gardner Museum (280 The Fenway, 617/566-1401, www.gardnermuseum.org, 11 a.m.–5 p.m. Tues.–Sun., $12 adults, $10 seniors, $5 students, free youth and children under 18) is filled with priceless European and American paintings. The most cherished work of art, however, may be the building itself, which is constructed around a plant-filled Italianate courtyard that may be the most pleasing indoor space in Boston.

The namesake socialite who built the Isabella Stewart Gardner Museum was known as something of a brilliant eccentric, who wore Red Sox caps with her ball gowns and scandalized polite society by posing for an eroticized portrait by John Singer Sargent. (On display in the museum, the portrait was exhibited only once in Gardner’s lifetime due to the wishes of her husband.)

The Isabella Stewart Gardner Museum keeps alive Gardner’s eccentric spirit by allowing any woman named Isabella free admission to the museum at all times.

Other works of art in the collection include Titian’s Europa, which may be the single most important work of art in Boston; Sargent’s dynamic El Jaleo; Boticelli’s Virgin and Child with an Angel; and an early Rembrandt self-portrait.

The building, which was also Gardner’s residence, has four floors of artwork organized as a living house museum, with some of the original typed labels still in place. Gardner’s will stipulated that nothing in the museum be moved, or else the entire collection would be sold and the proceeds donated to Harvard’s art faculty.

That requirement presented particular problems after the night of St. Patrick’s Day, 1990, when two thieves broke into the museum and cut thirteen paintings out of their frames. Among the priceless works of art stolen were two rare Rembrandt paintings, including a later self-portrait, and one of only about 35 Vermeers in the world. The theft, which some have called the largest art heist in history, still remains unsolved despite a $5 million reward offered by the museum; the frames for the stolen paintings still hang in a room called the Dutch Room.

Next door, a room called the Tapestry Room provides a beautiful background for periodic chamber music concerts. Ms. Gardner’s collection isn’t all that is on display at the museum; a small exhibition space hosts contemporary shows.

Not to be left behind in the Boston art museum renovation sweepstakes, the Gardner too plans a major expansion, which will include a new building adjacent to the museum designed by award-winning architect Renzo Piano, further increasing the space for contemporary work and traveling exhibitions. The renovation recently passed a major hurdle when the Massachusetts Supreme Judicial Court ruled that it was consistent with Gardner’s will and could proceed.