For the major metropolitan newspapers of the post-Civil War era, and especially the twentieth century, the best place to begin is:
For pre-twentieth century American newspapers, the two best collections are:
There are hundreds of freely available digitized newspaper collections. These collections vary wildly in quality, but to the historian hunting down evidence, quality isn’t always a top priority: if a collection has the source a historian needs, then he or she will happily use it, especially if it’s freely available online. Many states have their own digital newspaper collections, often developed in tandem with the National Digital Newspaper Program (NDNP). States that have partnered with the NDNP, but that have not developed their own separate collections, are not included here. This list is divided by region:
The main newspaper directories for North America were Rowell’s and Ayer’s. Published annually, these directories were actually intended as a guide for advertisers seeking information about publications in which to place advertisements. The titles changed frequently, and the two eventually merged in 1910. The directories are organized by state and then city. For each newspaper, the directory will typically include information like address, name of editor, frequency of publication, size of the newspaper, circulation numbers, political affiliation, and more. These directories are indispensable tools for researching American newspapers. Volumes through 1922 are available online. After 1922, the volumes are available in print only (at the History, Philosophy, and Newspaper Library).