The Polish striker added a second later as Chris Coleman's injury-hit Sky Blues made it five games without a win and Reading ended a run of four successive defeats.
Coleman said: "You're always going to be up against it if you go 1-0 down after 35 seconds, especially with a young team."
Reading gave his side due warning in the first attack of the game, Rasiak heading just over from a Jobi McAnuff cross.
But when McAnuff came up with a similar delivery seconds later, Rasiak was again free, this time heading just under the bar.
Man-of-the-match McAnuff doubled the lead nine minutes into the second half when he volleyed in from distance.
Freddy Eastwood gave Coventry hope - and their first goal in four matches - 10 minutes later when he drilled a low shot from 25 yards across Reading keeper Adam Federici and just inside his right-hand post.
But rather than inspire the home side, the strike only served to prompt Reading to move up a gear. Alex Pearce had a header cleared off the line by Jordan Clarke before Rasiak ensured it was game over 18 minutes from time.
McAnuff was again the dangerman, firing against a post for Rasiak to force home the rebound.
McAnuff followed up by going within inches of scoring his second, climaxing another defence-splitting run with a cross shot that just missed the far post.
And City's day was summed up with Clarke scooping the ball over when it looked easier to score.
Coleman added: "Freddy's goal was one of the few positives and we should have gone on after that."
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