General works on the Trinity can be found from BT 109 to BT 111.2. Scriptural sources for the doctrine are found in BT 112. The issue of "Procession" in the Trinity is found in BT 114. Comparative Religion works on the Trinity is found in BL 474. Anti-Trinitarian works are classified by specific heresies it is best to search by the heresy such as Arianism or Unitarianism. There is also a lot of material in the works of the early church fathers which is found in the BR 60 to BR 65 range.
Brief History of the Doctrine of the Trinity in the Early Church by 
The Cambridge Companion to the Trinity by 
The Deep Things of God by 
The Eternal Generation of the Son by 
Hilary of Poitiers on the Trinity by 
Richard of Saint Victor, on the Trinity by 
Theological Treatises on the Trinity by 
Trinitarian Theology after Barth by 
The Trinity by 
The Trinity in the New Testament by 
Which Trinity? Whose Monotheism? by 
