The hosts went behind to a Jonathan Stead goal, but Jobi McAnuff equalised and Noel Hunt added a second for Reading before the break.
Long beat James twice in the second half for his seventh and eighth goals of the season and his manager Brian McDermott said: "Shane has been suffering with flu this week and wasn't able to train until Christmas Eve,
"It wasn't just the fact that he scored two goals today, it was his all-round performance.
"He has had some criticism from the fans, but he knows I have got 100% trust in him."
Source: PA
Source: PA