References dealing with $\nabla f(p)$?

Question

What is the best reference you know dealing with gradient properties (related to maxima and minima) of functions from $\mathbb R^n$ to $\mathbb R$?

Recall the gradient of a function $f:U\longrightarrow \mathbb R$ defined on an open set $U\subset \mathbb R^n$ is given by:

$$\nabla f(p)=\left(\frac{\partial f}{\partial x_1}(p), \ldots, \frac{\partial f}{\partial x_n}(p)\right).$$

I have checked several books and I was not happy with their exposition.

Thanks.