I should have said the new book has one additional species since I don't know if the extra one is a moth or a butterfly.

Cabbage whites always have some black markings. You could possibly have the native Margined White, closely related. The summer forms are all white with no black markings. The caterpillars of the margined white eat crucifers. You would have to be near an undeveloped area to have these in the city. Otherwise, look more closely for black markings. The cabbage white caterpillar also feeds on nasturtium.

What kind of plant is it?

Looks like Eastern Comma Polygonia comma http://www.gardenswithwings.com/facts-info/ncommaquestionmark.html That little bright white mark on the underside looks like a comma on the "Eastern Comma" and a question mark on the "Question Mark".