The census is one of the genealogist's best friends. Sometimes, though, we see the forms on the screen or on microfilm and we're not quite sure what the headings are. Other times, we're not quite sure what information the enumerator was required to collect. Here are some resources that might be interesting and/or helpful to you.
● Enumeration Forms at The University of Minnesota's Population Center are a great resource for enumerator instructions because we can read a transcription of the actual instructions as the enumerators read and used them. The census questions are also available at this site. Dates available are 1850-2000. I found it interesting to compare the language in the enumerator instructions from early years to more recent years. Times change.
● A Century of Population Growth is a 1900 publication of the Bureau of the Census. It's a brief 2-page pdf image which discusses the value of and attitude toward censuses. The paper opens with "The results of a modern census have been accurately defined as a national account of stock. Early censuses were merely counts of inhabitants; additional facts relating to population were next secured; and the most recent step in census taking, especially in the United States, has been to include practically all lines of human activity. The modern census is thus the results of evolution."
● 200 Years of Census Taking: Population and Housing Questions, 1790-1990 is a 117-page publication from the U.S. Department of Congress, Bureau of the Census. It includes a table of contents; overviews of each census; images of the instructions to enumerators (as opposed to the transcriptions at the link above); some photographs; and a few cartoons. I believe that in this publication you will find the answer to any question you have about any U.S. census from 1790 to 1980 (excepting the agricultural census and unless it deals with handwriting and/or a particular name written on the census).
● The most recent publication is Measuring America: The Decennial Censuses from 1790 to 2000 which is available online in two parts. Part 1 includes title pages and table of contents. Part 2 includes information about each of the U.S. censuses, transcriptions of instructions to enumerators, and images of census pages. If you'd like to know what you'll find on the 1940 census, look at pages 62-66. Despite the title of this publication, information about the individual censuses ends with the 1990 census.
I think it's less than four months until the 1940 census will be available to the public. I probably won't find "new" ancestors in the 1940 census but it will be interesting to learn where my families lived, what their their employment was, who their neighbors were, etc.
The image above comes from the cover of Two Hundred Years of U.S. Census Taking.