The National Archives helps people with all kinds of research including genealogy. Just this last April they have published a 160 page guide to using the National Archives for your genealogical research. Timed to coincide with the digital release of the 1940 census, the guide is fully up to date. You can’t find it on Amazon, but it is available directly from the National Archives estore.
It is listed in WorldCat so you might be able to InterLibrary Loan it.
They also offer a 4 page handout on using the National Archives for Genealogy.
They also have a special starting page for genealogists:
Plus a playlist mostly for genealogists on their YouTube channel: