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.
In Epp-Bible Answers to Your Questions a 66 page booklet, Epp answers various questions on different Bible issues.
Steele – Love Enthroned is a work on what is love, its different aspects, biblical examples, etc. 23 chs.
Walker-The Doctrine of the Holy Spirit is a 7 chapter general work on the Holy Spirit with 9 appendices. He treats OT, person of the Spirit, work of the Spirit, HS in Christ, etc.
Bavinck The Holy Spirit’s Work of Calling and Regeneration is from the fourth and final full volume of Herman Bavinck’s Reformed Dogmatics
In Chuck Smith Person and Gift of the Holy Spirit, Smith focuses on the Holy Spirit living and working within the believer, as well as the person of the Holy Spirit.