My favorite section is a detailed list of all the esxtop counters and exactly what they mean and how they inter-relate.
For example, I had a question about what is the difference between the %USED and %RUN for a VM. On page 155 and 156 of this great document it gives you the specific formula for %USED, which is:
%USED = %RUN + %SYS - %OVLP
It also explains in the %RUN definition that on systems with hyperthreading enabled, the %RUN value does not account for hyperthreading. This can cause %RUN to be twice as large as %USED. But wait this can't be possible based on the formula - except in this case it is because %USED accounts for hyperthreading.
Another mystery of the universe is solved with the help of the vSphere Monitoring and Performance guide.