City had led through Jonathan Stead's 15th minute strike but Jobi McAnuff levelled and Noel Hunt had Reading in front just before half-time.
Long then grabbed a second-half brace to secure the points that keep Reading seventh, now only one point off the play-off places.
The Robins made the breakthrough when Steven Caulker found acres of space behind the home defence. The defender got to the ball before Adam Federici and attempted a clever lob. The ball bounced back off the crossbar allowing Stead to head in and earn a deserved lead.
Reading's improvement finally paid off as Jimmy Kebe whipped in a cross from the right wing for McAnuff to score at the second attempt, his header beating David James only to come back off the right post for him to rebound high into the net.
Hunt ensured that the Royals went in at half-time the happier side after excellent work from Long. The Irish winger crossed for Hunt to tap into an empty net. Long then calmy slotted a third into the left corner after 66 minutes. And The victory was sewn up as the Long seized on a pass from Hal Robson-Kanu and fired his second goal past hapless James with a minute to go.
Source: PA
Source: PA