The Mystery of the Trinity and the Mission of the Spirit, collection of 8 lectures in 2 parts given in Dornach, Oxford and London, 1922, by Rudolf Steiner (CW 214).
The entire book as a zip file.
Part 1: The Mystery of the Trinity
1. Lecture 1 (Dornach, July 23, 1922)
2. Lecture 2 (Dornach, July 28, 1922)
3. Lecture 3 (Dornach, July 29, 1922)
4. Lecture 4 (Dornach, July 30, 1922)
Part 2: The Mission of the Spirit
5. Meditation: The Path to Higher Knowledge (Oxford, August 20, 1922)
7. The Mystery of Golgotha (Oxford, August 27, 1922)
8. The Other Side of Human Existence (London, August 30, 1922)