"The online Gifford Lectures database presents a comprehensive collection of books derived from the Gifford Lectures." Some of the books listed here are not yet available full-text. Check HENRY for print availability.
Try browsing by subject (early church) or author (which lists their dates). Contains a lot of information about the early church (from Calvin College).
Access the 1917 edition of the Catholic Encyclopedia online, covering topics related to the Church's teachings, history, contributions, struggles, and much more.