  We produce an atlas of homogeneously reduced and calibrated low resolution
IRS spectra of the nuclear regions of nearby early-type galaxies (i.e. Es and
S0s, ETGs), in order to build a reference sample in the mid-infrared window.
From the Spitzer Heritage Archive we extract ETGs in the "Revised Shapley-Ames
Catalog of Bright Galaxies" having an IRS SL and/or LL spectrum. We recover 91
spectra out of 363 galaxies classified as ETGs in the catalog: 56 E (E0-E6), 8
mixed E/S0+S0/E, 27 S0 (both normal and barred - SB0) plus mixed types
SB0/Sa+SB0/SBa. For each galaxy, we provide the fully reduced and calibrated
spectrum, the intensity of nebular and molecular emission lines as well as of
the Polycyclic Aromatic Hydrocarbons (PAHs) after a template spectrum of a
passively evolving ETG has been subtracted. Spectra are classified into five
mid-infrared 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). A demographic study of mid-infrared spectra shows
that Es are significantly more passive than S0s: 46^{+11}_{-10}% of Es and
20^{+11}_{-7}% of S0s have a spectrum of class-0. Emission lines are revealed
in 64^{+12}_{-6}% of ETGs. The H_2-S(1) line is found with similar rate in Es
(34^{+10}_{-8}%) and in S0s (51^{+15}_{-12}%). PAHs are detected in
47^{+8}_{-7}% of ETGs, but only 9^{+4}_{-3}% have PAHs ratios typical of star
forming galaxies. Several indicators, such as peculiar morphologies and
kinematics, dust--lane irregular shape, radio and X-ray properties, suggest
that mid-infrared spectral classes are connected to phases of
accretion/feedback phenomena occurring in the nuclei of ETGs.
