A deposition is both "[a] witness's out-of-court testimony that is reduced to writing (usu. by a court reporter) for later use in court or for discovery purposes" and "[t]he session at which such testimony is recorded." Deposition, Black's Law Dictionary (10th ed. 2014).
This guide includes a sampling of the materials Jenkins has on depositions. For more sources, try searching the Jenkins' catalog for deposition*. 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 the procedures surrounding depositions. 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.