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. One example of a digital archive is the North American women's letters and diaries. This is an electronic resource that is available to you by searching CLIO and using your University ID (UNI).
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.
Another useful resource containing primary and secondary sources is Theological Commons maintained by Princeton Theological Seminary.
It is important when thinking of digital sources that there will be free sources, like those digitized and offered by the Library of Congress, and then there will be sources from commercial archives. The Alexander Street Press would be a commercial entity which provided the access for the North American women's letters and diaries.