Robert Shaw’s short 1 chapter work on God and the Holy Trinity is an excellent collection of observations and verses that defend the Trinity.
Teasley Holy Spirit and Other Spirits is about the Holy Spirit’s effect on the believer as wells as how other spirits (demons) try to affect mankind with specific unholy spirits.
Bunyan, John – Prayer is a work by the puritan about how to properly pray with the spirit, believing the promises of God.
Anderson Silence of God theodicy is about when God is silent, the silence of God, which in theology it is theodicy. Why doesn’t God immediately vindicate his children.
Berry Adventures in the Land of Canaan is a devotional work about Israel entering into the promised land as a parallel to our spiritual warfare towards our sanctification.
In Epp-Bible Answers to Your Questions a 66 page booklet, Epp answers various questions on different Bible issues.
Best- Diminishing Spirituality In Local Churches is a work by a pastor (Baptist I believe) in which he explores why our modern churches are less and less spiritual.