OP, do you mean the National Book Festival today (9/3) at the DC convention center? I looked online to see if there was a "DC Book Fair" but only get listings of lots of different, small events. The National Book Festival is a large event and it's a lot of fun but it's half over at this point in the day -- Still, if you see this: Have your teen hop online at the link below and scroll down to see if there are author talks of interest, you might still be able to catch someone your teen or you likes if you hurry!
https://www.loc.gov/events/2022-national-book-festival/
If you mean a different book event and I"m not seeing it online, apologies!