I got stuck on understanding stuff from Methods of molecular quantum mechanics written by R. McWeeny.
The content is:
It must be stressed that the representation property embodied in $\mathcal{A} \mathcal{B} = \mathcal{C}$ and $\mathbf{A} \mathbf{B}= \mathbf{C}$, with the matrix elements defined as in $ H_{ji}=\langle\phi_{j}|\mathcal{H}|\phi_{i}\rangle$, depends on the use of an orthonormal basis $ \left\{ \phi_{i} \right\} $. If the basis is non-orthonormal, with a metric $ \mathbf{S} $ defined as $ S_{ij}=\langle\phi_{i}|\phi_{j}\rangle $, then the "metrically defined" matrices ($ \mathbf{A}^{S}, \: \mathbf{B}^{S} , \cdots $, say) do not reflect the properties of the operators; when $\mathcal{A} \mathcal{B}= \mathcal{C}$ it is not generally true that $ \mathbf{A}^{S} \mathbf{B}^{S} = \mathbf{C}^{S} $.
[Matrices are written in boldface characters that represent the corresponding operators written in calligraphic style.]
I already studied some undergraduate-level Linear Algebra, but I'm not familiar with "metric" and the related notation superscipted $S$. From what material can I learn about this?