There are two Pennsylvania legal encyclopedias: Pennsylvania Law Encylopedia (published by LexisNexis) and Summary of Pennsylvania Jurisprudence 2d (published by West Group.) Both provide general coverage of Pennsylvania criminal law. The Pennsylvania Law Encyclopedia also covers Pennsylvania criminal procedure.
Legal Encyclopedias usually aren't detailed enough to be the final destination on an attorney's research trial, but they are often a good first stop for a researcher working with an unfamiliar area.