Here is a good reference for this
Michael Taylor: Pseudodifferential Operators, Princeton University PRess, 1981
In Chapter 12 it explains how to construct $f(A)$ when $A$ is elliptic selfadjoint of order $1$, $A\geq 0$, and $f$ is a smooth symbol of order $m$, i.e., $f$ is smooth and
$$ f^{(k)}(\lambda)= O(|\lambda|^{m-k}),\;\;\lambda\to\infty $$,
for any nonnegative integer $k$

