Looking for book or article that derives formula for estimating the "average heterozygosity"
$H=4N\mu$ is the formula you're trying to derive. 

It is an approximation coming from an infinite site model and applies to diploid populations only. For haploids, it becomes $H=2N\mu$. The value $4N\mu$ or $2N\mu$ is often also called $\theta$ as it is a value that appears in many places in the field.

There are a number of ways to derive it. It can be derived from coalescent theory or from classical fixation index stuff from Wright.

Any good intro book to population genetics should derive this formula. (see for example in Gillespie's book). You will find more book recommendation at Books on population or evolutionary genetics?.