An interrogatory is "[a] written question (usu. in a set of questions) submitted to an opposing party in a lawsuit as part of discovery." Interrogatory, Black's Law Dictionary (10th ed. 2014).
This guide includes a sampling of the materials Jenkins has on interrogatories. For more sources, try searching the Jenkins' catalog for interrogat*. The search results can be limited by adding additional terms to the search or by modifying the search to a specific library location, i.e. General Collection or Pennsylvania Collection.
Court rules often outline how evidence can be discovered and admitted in court as well as how discovery and interrogatories can be conducted. See the Pennsylvania Court Rules - State, Pennsylvania Court Rules - County, and the Federal Court Rules guides for how to access Pennsylvania and Federal court rules. Jenkins also has court rules for all 50 states and the District of Columbia. A search of the Jenkins' catalog for court rules and the jurisdiction should find the relevant materials.