Long's second-half double secured a third successive victory for Brian McDermott's side, although the Royals nearly threw away two points having worked so hard to break down Plymouth's stubborn resistance.
The home side dominated the first half without seriously threatening Plymouth keeper David Stockdale, with defender Ivar Ingimarsson twice heading wide from corners in a half of few chances.
Ironically, when Reading did finally make the breakthrough it came just after Plymouth had enjoyed their best spell of the game after twice going close in the opening five minutes of the second half having previously hardly threatened.
First Jamie Mackie forced Adam Federici to go full stretch to fingertip away his low shot with Andy Griffin completing the clearance to deny Kenny Cooper on the follow-up.
Then from the resulting corner, the unmarked Gary Sawyer seemed certain to score but saw his side-foot effort superbly held by Federici.
Within a minute, Reading were in front as Brian Howard's superbly weighted throughball on 51 minutes sent Long scampering clear and the Irish striker kept his cool to score for the third successive game.
Ten minutes later, Long should have doubled the lead after Howard again teed him up, but this time the striker drilled wide with only keeper Stockdale to beat.
It proved a costly miss as Paul Mariner's side were soon on level terms.
Kari Arnason's miscued shot turned into a useful pass and when the ball eventually found its way to Chris Clark, he pulled the ball across for skipper Carl Fletcher to drill through a crowd of players and in.
But Reading secured three priceless points in a frantic finale which saw Howard's free-kick scrambled clear and Brynjar Gunnarsson's shot hit a team-mate on its way towards goal.
However, Hal Robson-Kanu made amends in stoppage time after spinning clear of Reda Johnson and Clark, whose push handed Long the chance to drill home his fourth goal in three games from the spot.