Bit of an oddball, I don't think you can "learn" PCB design, I think you can only learn the tools that allow you to do PCB design, be that efficient or inefficient.
PCB design consists of component placement, routing placements, good use of planes, vias, and calculating current carrying track widths etc.
Clearances are one of the big issues as well as silk screen placement I have seen to be an issue from beginners, i.e. don't put silkscreen over the TH pads.
I think you need to have an understanding of good thermal relief also, not to put via's too close to a pad, or in the pad unless absolutely necessary as it removes heat away from the joint and will cause solder flow issues.
If you are designing for RF or high speed, make sure you take note of track lengths, and if you know the stackup of the PCB you can get a good feel for making impedance matched tracks.
High voltage needs separation from low voltage as it can spike across onto the LV side and ruin your day.
There are many considerations to laying out a PCB depending on what is on it and what it will be used for, and most can be collated and learned from the good people here and from Dave himself, I like to get myself a bit of a checklist going, and when I have got a chance take a look and see if I have kept to my own rules.