Thanks for using the PowerCLI Reference.
In chapter 15 we give an introduction to vSphere performance reporting with PowerCLI. But that only touches the surface I admit, the chapter was intended to give some intro to the concepts behind the PerformanceManager and how to use the data from PowerCLI.
The basic information about vSphere performance counters is documented in the PerformanceManager in the SDK Reference. But that of course doesn't explain which counters to use to investigate which performance problem.
In my PowerCLI & vSphere statistics – Part 1 – The basics post I provide a number of links to posts/articles that explain which counter to use for which characteristic of the environment.
One recent document I can highly recommend is the very recent Performance Best Practices for VMware vSphere® 5.1.