Fled to the hills is hyperbole. They are commuters who only come into the city to work. No one works on the weekends so the city would be relatively empty. Only the locals would remain (since they live there) to "man the pumps" which is just a metaphor to say "take care of things". The expression comes from the old fire fighting days where they would put a man on the pumps to pump the water while the others fought the fire. A best seller is a book that sells really well. News papers and magazines would a list of books that sold better than others. The ones that sold the best were called bestsellers. To prop something up means to support it or to keep it from falling. The easiest way to keep something from falling is to lean against something sturdier. In this case, these two books were leaning against John Adams bestseller. Not her period means (in this case) from a previous generation. One that someone from her generation or period would normally be unfamiliar with. I just read Karissa's definition of period. Go with that. It is correct. I hadn't considered that.

Commuters flee to the hills on the weekends, because commuters are people who have to go a far distance to get to their work every day. So they probably work in a city and live in the country, thus, the hills. Man the pumps means work the pumps, so having people there who will make sure the pumps keep running even though the commuters are home. Propped up against means it was leaning on it so it doesn't fall over. Yes, bestseller on John Adams means a book about John Adams, the president, that is selling very well, and many people are currently reading it. A bestseller BY John Adams would mean a popular book written by someone named John Adams, and couldn't be the president, because he's dead. Not her period means that she studies art, but she doesn't study the art of the period when the artist Rothko was making art.