Brazilian forward Ronaldinho threaded the ball through Sociedad's defense to Iniesta, who rounded goalkeeper Claudio Bravo before slotting in Barcelona's first goal in the 47th minute.
Then it was Ronaldinho again, supplying a pass to an unmarked Eto'o who raced forward, shooting past the advancing Bravo in the 88th minute.
Chances for both sides
Barcelona, even without injured midfielders Deco and Rafael Marquez, increasingly dominated its opponents until halftime, but Iniesta's goal just after the restart was followed by chances for both sides, with Sociedad forward Savio Bortolini shooting wide in the 54th.
A powerful shot by Eto'o was blocked by Bravo just two minutes later.
Ronaldinho, who had briefly left the field after he was hit in the face by Sociedad midfielder Victor Lopez, almost scored his 18th goal of the season with a curling free kick in the 72nd, only for it to fly over the crossbar.
Seven minutes later, he wasted a clear chance after a lazy dribble by Eto'o, while Lionel Messi raced past several Sociedad defenders before shooting narrowly wide.
Moments before Eto'o's goal, Savio had an opportunity to equalise against the run of play but shot over.
Sociedad, whose defeat ended a three-game winning streak at Anoeta stadium, stayed in 19th place, increasing fears it might lose its place in the top flight after 40 years.
Valencia v Deportivo La Coruna
|Deportivo Coruna's Pablo Alvarez, left, tangles|
with Valencia's David Silva [AFP]
Meanwhile, fourth-place Valencia kept its slim title hopes alive with a 2-1 win over Deportivo La Coruna, with midfielders Hugo Viana and David Silva scoring in each half for the 2002 and 2004 champions.
Valencia ended a six-game winless streak away from home despite playing without suspended captain David Albelda at Deportivo's Riazor Stadium.
Viana gave Valencia a 39th-minute lead when he delivered an angled volley from close range after Deportivo midfielder Aldo Duscher had dispossessed Silva.
Deportivo substitute Sebastian Taborda, who had entered the field only seconds earlier, scored a close-range equaliser in the 69th.
However, Silva won the game for Valencia five minutes later following a swift counterattack involving David Villa and Miguel Angel Angulo.
Barcelona, which is also challenging for the Copa del Rey, has 65 points with five games to play, while Sevilla has 61 points, with the two teams meeting on Sunday at Santiago Bernabeu Stadium. Valencia, which has five matches left, has 59 points.