You're book is a good idea. My hubby and I went once in January to see the Georgia O'Keefe exhibit. The National Zoo was empty and it is free. The panda was happily stripping leaves off a rather sizable bamboo stalk. Many of the animals were active because it was cold. I believe there may have been some snow on the ground.
We were also there to see the Olympic preliminary soccer game at RFK stadium. That was when our senator got us white house tickets and also had his intern give us a tour of the Capitol building.
I have been on there with school and scouting trips and wrapped up our Virginia civil war trip nearby and visited a friend in the city. I plan hotels by proximity to a subway station. I haven't seen the book yet but I sure it is a big help in one of the least expensive cities to visit.