The 1940 Census lists answers to several new questions never asked before including where they lived in 1935 and what was their income for the previous year.
This source is in its first recorded form. (ie: the original marriage record license)
[Sources & Evidence]
The National Archives and Records Administration (NARA) in Washington, DC and its Regional Branches, is the principal repository for records relating to the US federal government. These include the original paper copies of the federal census, all pre-World War I military records, Native American records, military service and pension records, naturalization records, ship passenger lists, land-entry case files, and homestead and bounty land warrant records. NARA's website is the place to begin research for federal records.
Probate records are often overlooked in some family histories. Sometimes they can bring down your research brick wall. They may show relationships, financial status and life stories of your ancestors.
When it comes to spelling variations, be creative. Often clerks and government officials were unable to correctly record the names given them by unschooled immigrants not familiar with languages used in their port of entry. The surname was written down as the official heard it and the immigrant accepted that as the official American rendering of his name.