The authors stay well clear of speculation about the jihadists aboard Flight 93 as to their target.
Yet I did not know before reading the book that firefighters were twice ordered to evacuate, once when word of Flight 93 was inbound came through and a little later when air traffic controllers lost track of a military flight. Those were among the most gripping passages - how the folks who could and could not evacuate reacted - in the book. The pucker factor was very high.
My reasoning about the Pentagon plane targeting the White House is that it flew down the Mall and came back around to the Pentagon rather than heading straight into the Pentagon, which would have hit near the main entrance.
If you've ever taken that approach to Reagan National down the Mall, it's really hard to spot the White House. It's off the Mall, surprisingly small, and surrounded by trees. So, my theory is the terrorists missed the White House and circled around to hit the Pentagon instead. If so, that would mean the two Washington planes were originally targeting the White House and Capitol. As awful as 9-11 was, it could have been even worse.