Another random thought:

A section on "stuff that's not in OTP, but many people end up using".

For instance, poolboy comes to mind, alongside the circuit breakers I mentioned in the other comment.

lager for logging.

At one point, there was a lot of stuff in the basho github repo that seemed pretty widely used.

It's been a bit since I've used Erlang so maybe there are some others that come to mind as "you're likely to see this included in a production Erlang system".