Conquering the USSR was one of Hitler's primary ideological objectives. Securing "Lebensraum" (Living Space) to the east of Germany was the long term goal of his foreign policy. War with the Soviet Union was 100% necessary as part of achieving this objective.
Now, the more complex question is not "why did Hitler Invade the USSR?", it is "Why did Hitler invade the USSR in 1941?" There are multiple factors, which I will summarize below:
- The Germans believed that the British were pinning their hopes of victory on the Soviet Union's entry into the war. By defeating the USSR decisively, the Germans hoped that the British would agree to a peace treaty and bring the war to an end.
- The Soviet Union was filled with agriculturally and industrially important resources. Food from Ukraine, Oil from Azerbijan and many other important raw materials would allow the Germans to compete with the resources of the British Empire and potentially the United States of America, should it ever join the war.
- The Soviet Union was judged to be far weaker in 1941 than it's paper strength implied. This was partially prejudice, but also partially based on the precedent set by the Soviet performance in the Winter War, wherein the far inferior Finish forces kept the Soviets at bay for an extended period of time and inflicted disproportionate casualties on the Red Army.
- Although Hitler was always worried about fighting a two-front war, he argued (with some merit) that because the British were unable to launch an attack in the foreseeable future on the continent, that Russia would be the only front as long as they could be defeated quickly.
- The Wehrmacht in 1940-41 was a highly efficient fighting machine with extremely high morale in the wake of it's victory over Poland and France.
In essence, Hitler judged that there would be no better time to strike than the present, and that by continuing to delay, it would allow the Red Army more time to reorganize, expand, and construct frontier defenses.
In terms of sources, any general overview of the Second World War will go into these points, ranging from Shirer's The Rise and Fall of the Third Reich, to documentary overviews like The World at War. For more specific information about the economic motivation, the ever popular Wages of Destruction: The Making and Breaking of the Nazi Economy by Adam Tooze goes into detail about this, as well as covering the desire for "Lebensraum" and historical-economic context of Germany going into 1939.