Spiritual Depression: Its Causes and Its Cure, Dr. Martyn Lloyd-Jones, 1965. Gently but firmly addresses potential causes including inability to receive forgiveness for all sins, not moving beyond past failures, seeing only bad and not good, not recognizing false teaching, lack of faith, not seeing trials correctly, etc. This older book stays centered on Biblical solutions. Very helpful.
Dagg Manual of Theology is a theology work in 2 volumes by J.L. Dagg a Reformed Southern Baptist. It is an extensive, very ample presentation of doctrines.
This is an extensive Bible Systematic Theology (Bible Doctrines book) from a conservative point of view.
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