The main search tools for finding books, articles, databases, archival collections, and more at Yale (and beyond) can all be found on the "Find, Request, and Use" page of the library's website. Here are a few quick refreshers, but be sure to visit the page for a full overview:
Archive Grid contains records for finding aids from over 1000 archival institutions. Start here when you are seeking to discover if there are any relevant archival collections for your research beyond Yale. The "Summary View" option is a good way to filter your search results after running a search. This is an open access resource/no login required.
Background or "reference" sources are a great place to start your research. Reference works include bibliographies, scholarly encyclopedias, dictionaries, handbooks, and other sources that provide overviews of topics and suggestions for further reading.
Link to the general collections of Cambridge University Press, including the Cambridge Histories and Cambridge Companions. Titles include:
Oxford Research Encyclopedia of American History
A scholarly encyclopedia, often really useful for a variety of U.S. history research topics. Each article has a discussion of the literature, including primary sources -- a great starting point for finding more sources. Examples of articles include:
Offers peer-reviewed annotated bibliographies on specific topics across varied subject areas. The Atlantic History and Political Science modules may be particularly helpful, but, depending on your research area, other modules may provide leads for your work, too.
Collection of all the constitutions of the world in English translation, with many supplementary materials.
A key resource for finding legal materials. Among its many collections, you'll find the useful "State Constitutions Illustrated" as well as "World Constitutions Illustrated," including:
Note: these aren't literally illustrated but instead comprehensive collections of constitutions, constitutional histories, and an array of related materials.
Lengthy chapters in the Oxford Handbooks usually provide helpful overviews of scholarly topics and historical literature, along with suggestions for further reading. A number of the handbooks may be helpful depending on your research topic, such as:
Available only in print at Law Library (earlier editions also available at Law Library)
A large collection of resources. The Wiley Companions will be especially useful -- for instance, A Companion to the American Revolution.
Navigate to "Humanities" and then "History" in order to find a detailed listing of titles by subfield.
Key background source is American National Biography Online
And potentially for reference articles as well as primary sources, Ancestry Library Edition
In addition to the online library catalog (Books+ and Orbis), reference sources, and footnotes in sources you've already found (etc.), subject-specific databases are another extremely helpful resource for finding secondary literature, including the latest scholarly journal articles in the field, and a key one for researching U.S. history including early America is:
Searching America: History and Life will allow you to check for the latest scholarly articles, reviews of books, citations to book chapters, and more in historical journals such as The William and Mary Quarterly, Journal of the Early Republic, and Journal of American History.