PowerShell v3 features updateable help. The help files are not quite complete at the moment but a recent update to the help that is available brought a couple of very useful files:
The first gives a good overview of WMI and the its structure. Good background reading.
The second gives a very good start for using WMI Query Language – WQL. It doesn’t cover everything – for instance associations are mentioned but it does give enough to work sensibly with WMI filters.
Both files are recommended reading