Martin Buber was a Jewish philosopher that wrote a very famous poetic psychology book titled: Ich und Du (I and Thou). It's a short book, around 150 pages that was published in 1923. It's a difficult read, especially since its German poetry translated into English, and is a difficult philosophical theology treatise that is comparable to Jonathan Edwards' Nature of True Virtue.
I first learned about Martin Buber in Paul Tillich's book Theology of Culture, where Tillich devotes a chapter to Buber's I and Thou. I'd actually recommend reading Tillich's chapter on Buber before reading Buber's I and Thou, or some sort of summary (like this one at stanford.)
(Martin Buber and Rabbi Binyamin)
Summary of I And Thou
Buber defines two primitive words: I-Thou and I-It. (And before I say more than I know, I recommend reading this work before repeating anything I say about Buber, as the preface of my edition warns, "it must be read more than once."). I-Thou is the primitive where knowledge of existence dwells, but there is constant danger of falling into the I-It primitive. It may be helpful to think of these primitives as single words rather than relationships.
The I-Thou is awareness of other beings, not by any universal Platonic form, or summation of attributes, but by dialectical relation with that very Thou entity. In knowing Thou is more than the summation of all It's attributes, but it is the very Thou. In knowing Thou, then we arrive at knowledge of ourselves defined in the I-Thou reciprocal relationship with Thou, because I only understand my own existence in direct relationship with that Thou as a I-Thou primitive correlation. Once the Thou is reduced to a universal, I am also dissolved into those same universals, and likewise, when Thou is reduced to It's attributes, then I am also reduce to non-existence in the I-It. So the true knowledge of being lies in only the I-Thou.
The Eternal Thou
The Eternal Thou is similar to Jonathan Edwards' Being In General (see his treatise: The Nature of True Virtue). The Eternal Thou is a monotheistic being from which all Thous are derived, but no one Thou is the Eternal Thou.
The plurality of I and Thou
Paul Tillich's discussion of I-Thou draws out the plurality of the Thou, because to know a Thou means to know all other Thou's in relation to that Thou, yet without falling into the bland universal It. So at the same time, I not only know myself by knowing the Thou of my I-Thou relationship. I know all others of my own I (think community), in relationship to all other Thous. So I believe this is where the Trinity must be inserted, here to make sense of the Eternal Thou, and this is where an individual Christian does not have a meaningful relationship with the Trinity apart from the collective church body of Christians.
Paul Tillich's Theology of Culture has a very useful analogy for explaining the two primary ways that philosophy engages God: meeting a stranger (i.e. Deism or Atheism) and overcoming estrangement (i.e. Pantheism or Materialism), which I learned about through Michael Horton's A Christian Faith: A Systematic Theology for Pilgrims on the Way. Buber's I and Thou provides a system of knowledge that doesn't collapse into either extreme of Deism or Pantheism.
Related: christian faith, I and Thou, Ich und Du, Jewish, Martin Buber, meeting a stranger, michael horton, overcoming estrangement, Paul Tillich, Philosophy, systematic theology, Theology of Culture