In addition to the resources above, though not a secret, I’ve found Google Books to be an excellent resource on many topics, especially of the theological nature.
I was happy to find that some pastors and teachers in previous generations published little books, treating specific topics in a dozen pages or so. Though, there are plenty of multi-volume works available too.
Lots of good stuff from the past few hundred years that is perfectly readable even to laymen.
If you decide to try it, use a web browser and after your initial search click “Tools” at the top and the use the drop-down to select “Free Google Ebooks” for library scans and other public domain books.
A lot of them are coming from the collections of those great old universities before they went completely pagan.
If there was a heresy being promoted, then there was a defense of orthodoxy published as well. Pamphlets were flying off the presses!