I would investigate the IPC documentation regarding all aspects of PCB design.
To be a good PCB designer you need to know:
How PCB are made - PCB Manuafcture
How PCB assemblies are put together - SMD lines, reflow soldering etc etc
Some basic electronics - I'd say to at least ONC standard
You are creating a physical entitiy that has to be manufactured and then populated, as well as the electronic aspect of the PCB design process, it is my stron belief that if you do not understand any one of the aspects you are lacking in your knowledge to design PCB's.
I'm lucky in that I started designing PCB's when there were such things as apprentices, I started on tape up designs, but more importantly before I started my first design I spent a few months learning how to assemble PCB's. This was straight after coming out of College.
Subscribe to
Printed Circuit Design andf Fabrication
and read through the articles past and present, it covers all aspects of what a PCB designer has to have some understanding off.