If you know the precise book title for a public domain work on church history, there’s a good chance you can find that book and read it on archive.org. For example, this week I was looking for an edition of Clement of Alexandria’s Stromata. I searched for Ante-Nicene Fathers on archive.org and soon found the book and the passage I needed to see.

Internet Archive: Digital Library of Free Books, Movies, Music & Wayback Machine.