Steinbeck has a lot of unerrated works.

Once There Was a War is his other major nonfiction work, it's a compendium of his columns as a war correspondent during WWII.

In Dubious Battle is my favorite of his novels. It's an early one, it's not as polished as his later works he's known for, but it's a solid story on it's own and fascinating to read in relation to the themes of The Grapes of Wrath. It's almost like a trial run for Grapes.