Conquest of the West (that is, of Western Europe and/or the US) was not really a serious consideration for Soviet leadership after the early 1920s, with the possible exception of right after World War II. This issue actually formed one of the major points of contention between Stalin and the more radical wing of the Communist Party, led by Trotsky, in the 20s; Stalin promulgated his doctrine of "socialism in one country," which held that Communists could and should build socialism within the borders of the USSR, relying on relatively indirect methods of trying to spread revolution elsewhere. Faced with the failures of revolutions elsewhere, many Communists embraced this philosophy as an opportunity to reach socialism through more incremental means.

After World War II, from what I understand, there was at least some thought given to invasion plans for Western Europe within the military, but I don't have the impression that they ever received any serious consideration. The Soviet leaders were ideologically committed, but they were also practical, and knew that such an invasion would be exceedingly reckless. They thought about strategy in terms of geopolitics, and they intended to spread Communism that way.

None of this is to say that the USSR didn't have spies or attempt "subversion" in the West; they certainly did. But these efforts were not intended as accompaniments to any kind of direct takeover. Just as the US (or any other country) was, they were interested in incremental political and military advantages, and in pressing those advantages through proxy wars and diplomacy.

Source-wise, I think biographies actually give a good sense of the overarching goals of the Soviet leadership (specifically, I can recommend Kotkin's on Stalin and Taubman's on Khrushchev). I'm not as knowledgeable about the actual cloak-and-dagger kind of stuff, so maybe someone else can chime in with that.