  We determine the energy-level shift experienced by a neutral atom due the
quantum electromagnetic interaction with a layered dielectric body. We use the
technique of normal-mode expansion to quantize the electromagnetic field in the
presence of a layered, non-dispersive and non-absorptive dielectric. We
explicitly calculate the equal-time commutation relations between the electric
field and vector potential operators. We show that the commutator can be
expressed in terms of a generalized transverse delta-function and that this is
a consequence of using the generalized Coulomb gauge to quantize the
electromagnetic field. These mathematical tools turn out to be very helpful in
the calculation of the energy-level shift of the atom, which can be in its
ground state or excited. The results for the shift are then analysed
asymptotically in various regions of the system's parameter space -- with a
view to providing quick estimates of the influence of a single dielectric layer
on the Casimir-Polder interaction between an atom and a dielectric half-space.
We also investigate the impact of resonances between the wavelength of the
atomic transition and the thickness of the top layer.
