  The twisted hexagonal boron nitride (hBN) bilayer has demonstrated
exceptional properties, particularly the existence of electronic flat bands
without needing a magic angle, suggesting strong excitonic effects. Therefore,
a systematic approach is presented to study the excitonic properties of twisted
AA' hBN using the Bethe-Salpeter equation based on single-particle
tight-binding wave functions. These are provided by a one-particle Hamiltonian
that is parameterized to describe the main features of {\it ab initio}
calculations. The Bethe-Salpeter equation is then solved in the so-called
excitonic transition representation, which significantly reduces the problem
dimensionality by exploiting the system's symmetries. Consequently, the
excitonic energies and the excitonic wave functions are obtained from the
direct diagonalization of the effective two-particle Hamiltonian of the
Bethe-Salpeter equation. We have studied rotation angles as low as
$7.34^{\circ}$. The model allows the study of commensurate and incommensurate
moir\'e patterns at much lower computational cost than the {\it ab initio}
version of the Bethe-Salpeter equation. Here, using the model and effective
screening of the Keldysh type, we could obtain the absorption spectra and
characterize the excitonic properties of twisted hBN bilayers for different
rotation angles, demonstrating how this property affects the excitonic energies
and localizations of their wavefunctions.
