The Met has a few more art books posted on line. Bruegel... Bonnnard
They are not as enjoyable as the hardcover ones, but the older ones are free. LINK (and the newer ones are not for download but you can order them on line).And the commentary often gives historical and cultural background for the art, including books on fashion or architecture.
For history buffs they also have books on the ancient world, such as Cunieform texts ancient Babylon or of course ancient Egypt.