“Secular” is generally contrasted with “regular,” which stems from “regula,” meaning rule. Regulars are those under a rule (i.e., religious), while seculars are those who are not. Thus a diocesan priest would be a secular priest, but so would a priest from the FSSP or other such fraternities.
I would definitely endorse the suggestion of Bishop Sheen’s “The Priest Is Not His Own.” Consider also anything written by Pierre Chaignon, S.J. A Jesuit, he specialized in the spiritual direction of priests, and his “Meditations for the Use of the Secular Clergy” is an overall excellent work.