I'll be interested to hear how you find the book.
Although the definition of the Trinity as such is always said to be a development, the Christian idea has always been of a Triune God. We have always been baptised 'In the name of the Father and of the Son and of the Holy Spirit' (Matt. 28:19-20). In the beginning was The Word, and The Word was God, as John tells us (John 1:1). We can only apprehend Him through the Incarnate Word, and through the action of the Holy Spirit.
As St. Paul says: 'we have not received the spirit of this world, but the spirit which is of God'. (1 Cor. 2:12).
Augustine tells us to 'seek His face always', and he wrote most movingly: 'I have sought you intellectually and I have argued much and toiled much. Give me the strength to seek you as you have caused yourself to be found ... When we do attain to you, there will be an end to these many things which we say and do not attain, and you will remain one, yet all in all, and we shall say one thing praising you in unison, even ourselves also being made one in you.'
In this is love, not that we loved God, but that He loved us and sent His Son to be the propitiation for our sins. (1 John 4:10)