ERIC - This database is useful for finding research that discusses textbooks from an educational perspective. Below is an example of how you might start your search. Feel free to add additional keywords to the third line to focus the search towards your individual topic...
America: History and Life - This history database can be used to find articles that discuss textbooks and readers from a historical perspective.
American Periodicals - This database allows you to search the full text of magazines printed between 1741 and 1900. Use this resources to examine historical discussions of textbooks and readers.
Periodicals Archive Online - This archive indexes articles back to 1770 and can be used to look for historicical discussions about textbooks and readers.
Google Books - Here you may find discussions and mentions of historic textbooks/readers. You may also find full text access to textbooks that have been digitized.
Overviews of the history of textbooks in American education, culture and politics.