A painting by Johannes Vermeer, showing a maid handing a letter to her mistress. The background is completely black. The lady has been writing something at a table covered in blue cloth, and now the maid has interrupted her. The lady touches her hand to her chin in wonder. The psychological interest of the painting is in speculating on what she is thinking, on what the import of the letter is, on how much the maid knows or approves: none of which Vermeer tells us, or really even hints at, though there is so much in it to think about. It is a supremely equivocal picture.

Almost all Vermeer's paintings are well-known images, as there are so few of them and each is precious; but this one gained a bit more exposure as the cover of Terry Eagleton's informative (and amusing) 1983 text Literary Theory. It's one of those pictures that seems just made for book covers, as it looks so expressive, so full of layers of interpretation begging to be read into it, if not read out of it.

The maid is standing behind the writing-table, dressed in plain stuff. The dullness of her clothing blends into the black background: her hair is quite black, so really can't be seen. The intense light of the picture doesn't seem to come from anywhere: unlike with most of Vermeer's paintings, there's no recognisable sense of an open window off to one side. There's just a bright illumination on her face and bare arms, and on the small white slip of paper she's holding out for her mistress.

Wherever the light is coming from, it's flooding over the seated mistress: face, hair, both arms, her work-table, and her brilliant costume, the golden robe edged with ermine that features in so many of his works. It's like a kind of revelation. It's like a spotlight in its edged intensity. It's as if it should be piercing her soul... a nineteenth-century moralist would have done a much worse, shallower painting with these ingredients, making it either a skewering of her conscience, of a vindication of her purity. But...

But we have no idea what the moral is. At least I haven't. And I'm sure we're not meant to: there are no symbolic fal-lals saying innocence or tryst or infidelity scattered around the background, as some other painters of those ages did. No. Nothing but enigma.

The mistress (the model who is in so many of the artist's paintings, probably his wife Catharina) is facing left, and somewhat away from us. We see her curled hair, her profile, but not much of her expression as such. Her mouth is slightly open. She looks a little surprised, though only a little; the effect is added to by her hand resting gently under her chin. It's hard to tell where her eyes are pointed, as the angle she's facing away is precisely enough to tantalise and to obscure, but it seems to be into the middle distance, as if thinking or remembering, and not at her maid. The maid is looking down at her, a dumpy face, but not unpleasant. Trustworthy and trusting? Triumphant and cunning?

His paintings don't have exact titles. My book calls it Servant Handing a Letter to her Mistress as well as Maid holding out a Letter to her Mistress and Mistress and Maidservant doing the Accounts. It's 92 cm x 78.7 cm, has only traces of a signature added later (but it's indubitably Vermeer), is oil on canvas, and is now in the Frick Collection in New York.

The earliest definite knowledge of it was when it was engraved in 1809 and sold the following year for 601 francs. It went for 460 francs in 1818. Vermeer was such an unknown back then that in 1937 it was sold as a work of Terborch's. Prices go up, and it went for 75 000 francs at Paris in 1889, and went on a tour of owners in St Petersburg, Berlin, London, and New York, until finally getting to Frick in 1919.

Source for provenance: Bianconi, The Complete Paintings of Vermeer, Rizzoli 1967 (Penguin 1987)