If you are off line and want to research in the Pennsylvania Archives, a good book to keep by your desk is “Guide to the Published Archives of Pennsylvania” by Henry Howard Eddy and Martha L. Simonetti. It was first written in 1949, but was updated in 1976 by the Pennsylvania Historical and Museum Commission. I found used copies on Amazon.com for not too much money. It gives a good description for the information covered in each Series. Few names of individuals will be found in this guide, but if you are looking for something specific, it is a good start.
The Pennsylvania Archives series is free on Footnote.com. This is very helpful if you are looking for records to join Lineage Societies or just researching your Pennsylvania ancestors. I find it rather daunting to locate records at times, so the book has been helpful narrowing my search and pointing me in a more profitable direction.
Do you have any books you are particularly fond of using in your research?