Booth, Wayne C., Gregory Colomb, Joseph M. Williams. The Craft of Research. 3rd ed. (Chicago: University of Chicago Press, 2008).
Turabian, Kate L. A Manual for Writers of Research Papers, Theses, and Dissertations. 8th ed. (Chicago: University of Chicago Press, 2013).
Primary sources do not always need to be in physical form. Primary, secondary, and tertiary sources can be in digital or electronic format.
Some collections of primary sources have been scanned and digitized for ease of use and to ensure preservation of the original materials. Sometimes access to these materials is free, but sometimes there is a cost (e.g., via a subscription). One example of subscription digital archive is the North American women's letters and diaries. This is an electronic resource that is available to you at not cost by searching CLIO and using your University ID (UNI). Columbia University Libraries pays for this subscription. Many of these digital archive collections can also be found via Google, however we suggest starting with CLIO because many of these materials and sources are subscription-based and require a login. If Columbia University Library subscribes, they will be free to you via your UNI.
Here are two examples of open access (freely available) materials: Theological Commons maintained by Princeton Theological Seminary, which includes primary and secondary sources. Another open access (freely available) portal to sources is the Library of Congress Digital Collections and Services page,