As you touched on a little in your question there were several goals in the implementation of the Marshall Plan. First it is worth noting that the plan came about entirely due to George Marshall and his conclusions after traveling through post-war Europe. Without his status and reputation it would have had a much harder time passing and probably would have ended looking a lot different.

The first reason to discuss has to be the spread of communism. The American political elite was shocked at the Soviet domination of Eastern Europe post war and was terrified that communism would continue to spread, both because it threatened American economic interests in the area and because it was seen as a threat to America itself. France and Italy both came close to electing communist led governments in this period and Greece was fighting a civil war as well. There was a strong belief among the Americans that support or sympathy for communism was associated with poverty and want. Thus the solution was obvious, though it should be noted they did not just dump money on these countries but actively tried a variety of techniques, like the letter writing campaign from Italian Americans to their relatives back home or the importation of American miles to rural Greece. The view that the plan was fueled by red fear cannot be discounted.

On the other hand there is good evidence that Marshall personally was horrified at the conditions in Europe and genuinely wanted to help. The view that many Americans felt the same way is entirely valid (there are very interesting opinion polls showing American attitudes in the postwar period I can recommend checking out). While Americans were coming to see the Soviets as the new enemy they still saw countries like Britain and France as brothers in arms. The humanitarian urge should not be viewed cynically and can also be seen in other American actions like the extension of aid to Japan and their strong support of the new UN. In our current climate it might be hard to imagine but many Americans thought that their government should work to prevent another war and help those who were suffering.

The economic aspect is another important one, if slightly obvious. The war had pulled America out of the last vestiges of the Great Depression and now many factories were switching from tanks and battleships back to cars and radios. America needed markets for these goods and if German citizens were living in ruins and struggling for food they wouldn’t be able to buy anything American. This is arguably the same reason post World War One isolationist America still took time to sign the Dawes and Young plans to loan massive sums of money to a Europe they apparently wanted nothing to do with, it was simply good business.

There are other reasons too. Truman was wanting to highlight his foreign policy record and seem like a strong leader going into a seemingly impossible re-election campaign. Democrats wanted clear evidence to show they were strongly anti-communist as they were fielding more and more abuse from across the aisle, especially after the loss of China. One can also see a quid pro quo as countries who received economic aid were more likely to join NATO.

As for what was the most important reason there is really no easy, non-biased answer and five different historians will likely give five different explanations. I personally find the anti-communist argument the most likely with the added bonus of helping American business but whichever you choose will likely come from your own view of America in this period.

As for reading recommendations I have some hesitancy in offering anything too strongly as opinions still run strong on this topic and the beginning of the Cold War is a mess of differing opinions and contradictory views. What you will appreciate really depends upon where you sit on the orthodox-revisionist historiography always scale. If you want same safe post-revisionist middle of the road type stuff I would recommend John Lewis Gaddis. If you want a sense of the extremes you could try William A Williams for the revisionists and Thomas Bailey for the orthodox though be aware some of these (especially Bailey) are quite dated now.