The victory has lifted Roberto Di Matteo's side to the top of the Premier League table.
The hosts dominated the opening stages and came close when Eden Hazard dragged his shot wide.
The Blues then put together a fluid passing move that sent Ramires through on goal. The midfielder could have squared it to the unmarked Torres but instead opted to have a go, only to see his shot saved by Adam Federici.
They eventually took the lead on 18 minutes when Frank Lampard converted from the penalty spot after Chris Gunter had fouled Hazard.
Guthrie then gave the visitors the advantage just minutes later as his free kick was fumbled by goalkeeper Petr Cech into his own net.
Chelsea had an early chance to equalise at the start of the second half but Branislav Ivanovic sliced his shot high and wide.
Reading then defended solidly as Chelsea poured forward, with Juan Mata and Hazard both having their shots blocked.
However, they found a way through the opposition defence in fortunate circumstances on 69 minutes as Gary Cahill's speculative drive somehow went past Federici.
The turnaround was complete with just under 10 minutes to play thanks to Torres's strike. However, replays showed that the striker was clearly offside when he converted Ashley Cole's cross.
The Blues sealed the three points deep into injury time when Hazard teed up Ivanovic to slot into an empty net after Reading goalkeeper Federici had gone up for a corner.