  We consider a line-of-sight communication link between two holographic
surfaces (HoloSs). We provide a closed-form expression for the number of
effective degrees of freedom (eDoF), i.e., the number of orthogonal
communication modes that can be established between the HoloSs. The framework
can be applied to general network deployments beyond the widely studied
paraxial setting. This is obtained by utilizing a quartic approximation for the
wavefront of the electromagnetic waves, and by proving that the number of eDoF
corresponds to an instance of Landau's eigenvalue problem applied to a
bandlimited kernel determined by the quartic approximation of the wavefront.
The proposed approach overcomes the limitations of the widely utilized
parabolic approximation for the wavefront, which provides inaccurate estimates
in non-paraxial deployments. We specialize the framework to typical network
deployments, and provide analytical expressions for the optimal, according to
Kolmogorov's $N$-width criterion, basis functions (communication waveforms) for
optimal data encoding and decoding. With the aid of numerical analysis, we
validate the accuracy of the closed-form expressions for the number of eDoF and
waveforms.
