PRR Position Light Signal System - By the Philadelphia Chapter of the Pennsylvania Railroad Technical & Historical Society.
Pennsylvania Railroad Signal Drawing Symbols - Based on PRR Document S-601-A, 1935 - By Henry J. Sundermeyer.
Pennsylvania Railroad Position Light Signal Rules - Based on 1956 Book of Rules - Excellent Java app that demonstrates rules vs. signal aspects. By Henry J. Sundermeyer.
NORAC Signal Aspects Chart - Dated 1993, but still is a reference for Pennsy PL signal aspects!
The Position Light - A blog devoted to explaining the ins and outs of North American railroad signaling, past, present and future. Some excellent PRR coverage!