I haven't read the book, but I believe what he says is that the 'trinity' is not a composite - i.e. you don't need the Holy Spirit, Jesus AND God (father) to become God - i.e. Jesus is God, Holy Spirit is God and God is God - God has manifested himself in these three forms.
I base this on the composite VIDEO signal:
TV was first black and white. So, TV stations only had to transmit the levels of white (black is zero - for arguments sake -some signals were inverse, i.e. white was zero).
Anyway, when Colour was introduced there had to be a way to transmit color as well as be compatible with back and white TVs (people would not watch colour if they had to throw away their B&W TVs, right?).
So, the method was to send the normal black and white signal (called luminance) and send the colour information (called chrominance) 'separately' so that black and white TVs would understand the luminance but colour TVs aware of the crominance would 'look' for it and process it. If a chrominance signal was not availabe the colour TV would run in B&W mode. The 3rd part of the signal is the sound.
So, in composites, the colour signal alone is useless, the B&W signal alone is useless and the sound signal alone is useless (i.e. you don't get the full picture).
But with God, each of the holy trinity are independent but are also one. You get the total 'picture' either independently or as a trinity.
I am not saying St. Thomas Aquinas is right (I believe his analysis is meritorious), what I am trying to do is breakdown the explanation of (possibly) what he meant or to give you a better understanding...
Trust this helps.