Suggest me some good mid-20th century American literature

My wife's all-time favorite book has been Steinbeck's East of Eden. Recently she discovered The Night of The Hunter and she loved it. Now she's reading Betty Smith's A Tree Grows In Brooklyn, which became her new favorite. Oh? I see a pattern here. Even though she mostly reads modern books like fantasy and YA, she particularly seems to enjoy the atmosphere and style of books written in that specific period (& by US writers?). Something about those books is 'tasty', and they hit the spot, she says. Atmosphere over plots, perhaps?

I could get her more Steinbeck books as a gift, but that's an obvious choice. What other hidden gems are there, written in around 1940~50s ish that have a good, tasty, specific atmosphere? Thanks much.