This study reports the quasiparticle band gap, excitons, and highly anisotropic optical responses of few-layer black phosphorus (phosphorene). These materials exhibit unique many-electron effects, with electronic structures that are dispersive along one dimension, leading to enhanced self-energy corrections and excitonic effects. Few-layer black phosphorus absorbs light polarized along the armchair direction and is transparent to light polarized along the zigzag direction, making it a viable linear polarizer. The number of phosphorene layers controls the material's band gap, optical absorption spectrum, and anisotropic polarization energy window. Phosphorene is a direct band gap semiconductor with promising applications. Its unique excitonic effects have not been observed in other 2D structures. The band gap of phosphorene depends on the number of stacked layers, and the optical spectra and excitonic effects can be controlled by the number of stacking layers. First-principles GW-Bethe-Salpeter Equation (BSE) simulations were performed to study the QP band gap and optical spectra of few-layer and bulk black phosphorus, revealing significant many-electron effects. For monolayer phosphorene, the self-energy correction increases the band gap from 0.8 eV to 2 eV, and the lowest-energy optical absorption peak is reduced to 1.2 eV due to a large exciton binding energy. The optical absorption spectra of monolayer, bilayer, trilayer, and bulk phosphorene structures show that phosphorene strongly absorbs light polarized along the armchair direction and is transparent to light polarized along the zigzag direction. The optical absorption peaks and exciton binding energies are affected by the same mechanisms and follow similar scaling laws with the stacking layer number. The optical absorption spectra for light polarized along the zigzag direction show a different absorption energy range compared to the armchair direction. The anisotropic optical response of phosphorene makes it a natural optical linear polarizer, suitable for applications in liquid-crystal displays, three-dimensional visualization techniques, (bio)-dermatology, and optical quantum computers. The polarization energy window is tunable through a wide range, and the high-end of the polarization window is nearly fixed at 2.8 eV, while the low-end can be reduced from 1.1 eV down to 300 meV by adjusting the layer stacking number. This frequency range is very exploitable for applications, covering the infrared and near-infrared regimes. The anisotropic optical response can be used to identify the orientations of few-layer black phosphorus in experiments. The study also shows that the many-electron effects are rooted in the vast vacuum surrounding the isolated system and are not significantly affected by small changes in interlayer distance. The layers of bulk black phosphorus, on the other hand, do not interface with a vacuum, and their excited state properties are sensitive toThis study reports the quasiparticle band gap, excitons, and highly anisotropic optical responses of few-layer black phosphorus (phosphorene). These materials exhibit unique many-electron effects, with electronic structures that are dispersive along one dimension, leading to enhanced self-energy corrections and excitonic effects. Few-layer black phosphorus absorbs light polarized along the armchair direction and is transparent to light polarized along the zigzag direction, making it a viable linear polarizer. The number of phosphorene layers controls the material's band gap, optical absorption spectrum, and anisotropic polarization energy window. Phosphorene is a direct band gap semiconductor with promising applications. Its unique excitonic effects have not been observed in other 2D structures. The band gap of phosphorene depends on the number of stacked layers, and the optical spectra and excitonic effects can be controlled by the number of stacking layers. First-principles GW-Bethe-Salpeter Equation (BSE) simulations were performed to study the QP band gap and optical spectra of few-layer and bulk black phosphorus, revealing significant many-electron effects. For monolayer phosphorene, the self-energy correction increases the band gap from 0.8 eV to 2 eV, and the lowest-energy optical absorption peak is reduced to 1.2 eV due to a large exciton binding energy. The optical absorption spectra of monolayer, bilayer, trilayer, and bulk phosphorene structures show that phosphorene strongly absorbs light polarized along the armchair direction and is transparent to light polarized along the zigzag direction. The optical absorption peaks and exciton binding energies are affected by the same mechanisms and follow similar scaling laws with the stacking layer number. The optical absorption spectra for light polarized along the zigzag direction show a different absorption energy range compared to the armchair direction. The anisotropic optical response of phosphorene makes it a natural optical linear polarizer, suitable for applications in liquid-crystal displays, three-dimensional visualization techniques, (bio)-dermatology, and optical quantum computers. The polarization energy window is tunable through a wide range, and the high-end of the polarization window is nearly fixed at 2.8 eV, while the low-end can be reduced from 1.1 eV down to 300 meV by adjusting the layer stacking number. This frequency range is very exploitable for applications, covering the infrared and near-infrared regimes. The anisotropic optical response can be used to identify the orientations of few-layer black phosphorus in experiments. The study also shows that the many-electron effects are rooted in the vast vacuum surrounding the isolated system and are not significantly affected by small changes in interlayer distance. The layers of bulk black phosphorus, on the other hand, do not interface with a vacuum, and their excited state properties are sensitive to