What were the popular subjects in American literature in the 1940s?It would be wonderful if you could simply list the subjects and give as much example as possible.
That's kind of a tricky time in American Literature--first of all you get quite a few disaster narratives (mostly in the naturalist mode) during the depression. Actually quite a few graphic novels (comic books) with clearly defined heroes and villians were popular too at that time. Then after the war you get the Lost Generation--modernist who felt desolation, hopelessness, after the war. Read Eliot's "The Wasteland" or "The Hollow Men" for examples.
"The Race Question"
This isn't a complete answer, but the 1940s was a period when the Chicago Black Renaissance was going full steam (Richard Wright published Native Son in 1940, and Gwendolyn Brooks wrote her first book of poetry, A Street in Bronzeville). Many of these writers were concerned with depicting black urban life; Richard Wright in particular is also exploring the possibilities that the Communist party's activism hold for African American rights (the Communist party had a strict anti-racism policy, and was a pretty novel arena for many, as white and black Americans organized/worked together as equals). [By the way, later, Richard Wright quite publicly breaks with the Communist party.] The forties marks an important turning point in African American approaches to dealing with racism: before the war, Af Am newspapers focused on developing a strong (but separate) Af Am community: the goal was to gain enough rights in order to be able to live their lives, earn money, etc. Black soldiers fought in World War II, however, so after the war, there's a marked change in that African Americans begin demanding /equal/ rights, rather than "enough."
"Soldier experiences in WWII"
The forties in general are marked by opposing tensions. On the one hand, the world is reeling from the atrocities of fascism in Europe; many Europeans have fled to the U.S. in the face of Nazi persecution. So there's a sense of horror and disillusionment there. At the same time, the war industry pulls the U.S. out a long and difficult economic depression, and as an Ally in WWII, the U.S. becomes a world superpower. So there's a great deal of optimism and enthusiasm. When soldiers return, you get the baby boom (Dr. Spock first published in the 40s), and due to the GI Bill, many more people go to college. People are pretty fascinated by the war and the experience of soldiers-- one of the big books from the 1940s is Norman Mailer's The Naked and the Dead, which depicts soldiers in combat with pretty gritty realism; the novel is characterized by a heavy use of military lingo and strong, slangy talk between soldiers.
Just a note: msbrenner is correct that after the war, the Lost Generation was a group of modernist expatriate writers who were disillusioned with the carnage they had just witnessed-- but this was after the FIRST world war, in the 1920s, not in the 1940s.