I present an Atlas of homogeneously reduced and calibrated low resolution Spitzer-IRS spectra (~5 - 38 micron) of the nuclear regions of early-type galaxies (i.e. Es and S0s, ETGs) in the Revised Shapley-Ames Catalog of Bright Galaxies (RSA). The Atlas considers 91 galaxies (out of 363 classified as ETGs in RSA) with SL+LL IRS modules available in the Spitzer Heritage Archive: 56 E (E0-E6), 8 mixed E/S0+S0/E, 27 S0 (both normal and barred - SB0) plus mixed types SB0/Sa+SB0/SBa. Atomic and molecular emissions are measured as well as Polycyclic Aromatic Hydrocarbon (PAHs) complexes, after a template spectrum of a passively evolving ETG has been built and subtracted. All spectra spectra fit into five mid-infrared (MIR) classes, ranging from AGN (class-4) and star forming nuclei (class-3), transition class-2 (with PAHs) and class-1 (no-PAHs) to passively evolving nuclei (class-0). I discuss a demographic study of the MIR spectral classes. I use several indicators, such as peculiar morphologies and kinematics, dust--lane irregular shape, radio and X-ray properties, to argue for MIR spectral classes are connected to phases of accretion/feedback phenomena occurring in the nuclei of ETGs.