One example I can think of is Whose Names are Unknown, by Sanora Babb. She had a publishing deal for the book but it was shelved when Grapes of Wrath came out, as the publisher didn't think the market could support two books with similar settings and themes. It has since been published, relatively recently in 2004 (it was written in the 1930s), by a university press and in my opinion is better than Grapes. It's also very likely Steinbeck used her notes when writing Grapes of Wrath.