  We present a tight-binding (TB) model and $\mathbf{k\cdot p}$ theory for
electrons in monolayer and few-layer InSe. The model is constructed from a
basis of all $s$ and $p$ valence orbitals on both indium and selenium atoms,
with tight-binding parameters obtained from fitting to independently computed
density functional theory (DFT) band structures for mono- and bilayer InSe. For
the valence and conduction band edges of few-layer InSe, which appear to be in
the vicinity of the $\Gamma$ point, we calculate the absorption coefficient for
the principal optical transitions as a function of the number of layers, $N$.
We find a strong dependence on $N$ of the principal optical transition
energies, selection rules, and optical oscillation strengths, in agreement with
recent observations \cite{Bandurin2016}. Also, we find that the conduction band
electrons are relatively light ($m \propto 0.14-0.18 m_e$), in contrast to an
almost flat, and slightly inverted, dispersion of valence band holes near the
$\Gamma$-point, which is found for up to $N \propto 6$.
