Let $P:\Gamma(E)\rightarrow\Gamma(F)$ be an elliptic partial differential operator, with index $=0$ and closed image of codimension $=1$, between spaces $\Gamma(E)$ and $\Gamma(F)$ of smooth sections of vector bundles $E\rightarrow M$ and $F\rightarrow M$ on a Riemannian manifold $(M,g)$ without boundary.

*Question*:  What is the elliptic operator's associated Green's operator?  

More precisely, $(M,g)=(\mathbb{S}^m,g)$ be the unit $m$-sphere with constant curvature =1 metric $g$, so $\text{Ricc}(g)=g$.  Also let $E=F=S^2\mathbb{S}^m$, the space of $2$-covariant tensors on $\mathbb{S}^m$.  Consider the operator:
\begin{align*}
E:\Gamma(S^2M)&\rightarrow\Gamma(S^2M)\\
h&\mapsto Eh_{ij}=\frac{1}{2}g^{kl}(\nabla_i\nabla_jh_{kl}+\nabla_k\nabla_lh_{ij}).
\end{align*}
The symbol is:
$$
\sigma_E(\xi)h=\frac{1}{2}g^{kl}(\xi_i\xi_jh_{kl}+\xi_k\xi_lh_{ij}).
$$
We can show that $E$ is elliptic with index $=0$ and closed image of codimension $=1$.  

*Question*:  What is the Green's operator of $E$?

*Reference request*:  A good reference on Green's operators for elliptic partial differential operators would be welcomed.