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.