Codicibus nondum editis collectae, aut scriptae, digeste iuxta seriem annorum Christianae Epochae et Animadversionibus illustratae (Lives of the Sicilian Saints from ancient Greek and Latin Monuments and for the most part from unpublished manuscript codices or writings, sorted by years of the Christian Epoch and explained with observations).

[3] This was composed according to a plan originally appearing in his Idea operis de vitis sanctorum Siculorum, whose full title was Idea operis de vitis siculorum sanctorum famave sanctitatis illustrium Deo volente bonis iuvantibus in lucem prodituri (Report of the works and lives of the Sicilian saints whose notable sanctity has been brought to light in good works by God's will).

In September 1614 he was appointed head of the Casa Professa and after a short time became Preposto of the Collegio del Gesù Grande, quickly improving the financial situation of the College in Palermo, which had fallen heavily into debt.

During this time he finally managed to publish Idea operis in 1617, but Vitae Sanctorum Siculorum and Isagoge ad Historiam sacram siculam (Isagoge ad historiam sacram Siculam, ubi tam veteris Siciliae impiae superstitiones, quam verae fidei in eadem insula initia, propagatio et augmenta, Siculorum in religionem Christianam ardor et in ea constantia, aliaque hujus argumenti, eruditione copiosissima, et singulari methodo exponuntor[6] remained incomplete on his death.

[7][8] The book is still fundamental to Sicilian historical studies, consisting of two folio volumes totalling 825 pages and containing 200 lives, panegyrics, sermons, accounts of relic-translations, hymns and other texts relating to 120 saints, alongside Gaetani's own commentary.