The First Seven Ecumenical Councils (325-787): Their History and Theology (Theology and Life Series 21)

Like Fr. Sparky, I didn't read specific "discernment" books, but I was engaged in spiritual reading to help me know Christ better (as well as prayer and service). I don't believe I had read the whole NT before I applied to seminary, but I had done a deep dive on a single gospel, as I know I had read LT Johnson's commentary on Luke. I also read Leo Davis's First Seven Ecumenical Councils. This both helped me know Christ better through better understanding the Church's teaching on his nature, but also by seeing how his life was reflected in that of the saints. I also read a biography of the founder of the particular religious community I was discerning joining (Holy Cross). Nowadays, I would also recommend that people read the Church's authoritative expression of its own identity (Lumen Gentium) and the nature of priesthood (Presbyterum Ordinis).

I've always found Leo Davis, The First Seven Ecumenical Councils to be a great book to start with. The author is a Roman Catholic (I'm Orthodox so I was trying to be aware of bias). The book is informative without being too technical. You can by it from Amazon or view it on Internet Archive. There are even free pdf versions floating around the internet. https://www.amazon.com.au/First-Seven-Ecumenical-Councils-325-787/dp/0814656161/ref=sr_1_1?crid=3EADL53X7UVKU&dib=eyJ2IjoiMSJ9.pZlbH7gKWVxMAvQYkdETkobpdiLpWmaeELEpghMJQEyArdfa0q5xA39C34HC_KAM.h57mjA9VHHoxFXFkJNbKYq2jpov86oc5BoGINNxEixQ&dib_tag=se&keywords=Leo+davis+first+seven+Ecumenical+Councils&qid=1781419071&s=books&sprefix=leo+davis+first+seven+ecumenical+councils+%2Cstripbooks%2C245&sr=1-1 https://archive.org/details/firstsevenecumen0000davi