The paintings become mysteries, enticing and deserving of attention precisely because there are so few Johannes Vermeer (1632-75) is doubtless a blockbuster artist, but the Frick’s exhibition, Vermeer’s Love Letters, is the size of a postage stamp – or, maybe more fittingly, a wax seal. The Frick has three Vermeers of its own, but only one made the cut: “Mistress and Maid” (c. 1664-67). From the 37 known works by the artist, the museum has only borrowed two to bolster the show: “Woman Writing a