As in so many of Vermeer's apparently quiet, dignified interiors, there is implied movement and action underlying emotional turmoil. The mistress is in the act of writing a letter when her maid, body pushed forward in familiarity and forwardness, brings her another. The mistress' hand flies to her face in consternation. It is not the contents of the letter that concerns her, since she has not yet opened it, but the identity of the sender -- a significance not lost on the maid.
The figures are especially luminous and three-dimensional against the dark background. Compositionally, the two faces and the pen form a triangle, with the mysterious letter at the center of mass.
The mistress is dressed in the usual ermine-trimmed yellow jacket, which can also be seen in A Lady Writing, Woman with a Lute, Woman with a Pearl Necklace, The Love Letter, and The Guitar Player. In this painting, it is rendered exquisitely, the folds articulated, the fabric with delicate satin sheen, and the fur soft and natural.