Dagg’s Work, Manual of Theology is a full fledged systematic theology from a Reformed perspective. He was Baptist.
Dr. Luginbill’s work, Angelology, the Study of Angels, is a single chapter work on Angels. Although brief, it is quite well documented on the specifics of the doctrine of angels.
The Bible itself reveals those doctrines that are essential to the Christian faith. They are 1) the Deity of Christ, 2) Salvation by Grace, 3) Resurrection of Christ, 4) the gospel, and 5) monotheism. These are the doctrines the Bible says are necessary.
This small article could very well determine whether you are in or out of the Christian Faith
Litmus Test: You have to believe items 1-5 to be in the Christian Faith.
Christian Titles has 50 chapters, each one devoted to one of the descriptions Scripture gives to the saved people. These are studies into how they apply to Christians.
In Horatius Bonar Truth and Error he looks at the contrary between the two elements, truth and error.
Doctrinal Books and Articles by Arthur Pink
John Owen’s work on the Gospel Grounds and the Evidences of the Christian Faith from the perspective of God’s Elect.