hello Mr Disney,
the best way to learn PnR is to go through the Tool Mannuals.SoC encounter is one of the best tool in industry for Netlist to GDS flow and its documentation also is the best so please go through that.
Try to corelate your experience with new activity.Coz here (digital PD) you'll be playing with gates and other blocks not at the transistor level.
You can read book by Wayne wolf he has explained some basics of PnR in different chapters.
Also ASIC Design by S.M.J Smith can help you
regards,