(Click for much bigger version)
“I have tried to destroy the picture of the most beautiful woman in mythological history as a protest against the government for destroying Mrs Pankhurst, who is the most beautiful character in modern history,” Richardson said at the time, though she admitted in 1952 that another reason was simply that she “didn’t like the way men visitors gaped at it all day long”. (See more here.)
It's the painting's combination of sexiness and restraint, I suspect, that has seen it so widely used. Here are just a selection of covers which feature it, and this excludes the many other Velázquez-focused books which also make use of it.
The second book featured above, The King Amaz'd by Gonzalo Torrente Ballester, is a wonderful and strange novel: Spain is in trouble, struck by incursions from Hell, because the King desires to see his wife naked. Meanwhile, a pure-hearted priest who is able to chat with Satan without becoming corrupted, meets up with the Devil to sort out what can be done. Beautifully written, and obeying its own strange internal logic, it's a beautiful book.