Let's just start by taking a sentence as an example. Shortly before going to Giovanni's apartment in Nation, David says,
I ached abruptly, intolerably, with a longing to go home; not that hotel, in one of the alleys of Paris, where the concierge barred the way with my unpaid bill; but home, home across the ocean, to things and people I knew and understood; to those things, those places, those people which I would always helplessly, and in whatever bitterness of spirit, love above all else. (1.3.128)
At first glance, this appears to be what is called a "loose" sentence in which the main idea – David's longing to go home – is located at the beginning, and everything else is largely ornamental. Yet, as the sentence moves on, we see David clarifying and expanding upon his initial idea. The reason he wants to go home is to find "things and people I knew and understood." By the time the sentence reaches its conclusion, it has regained momentum, built up enough steam to re-make the main idea, or perhaps to create a new one.
At the end of the sentence, we learn that the reason he wants to go home is because there he will find "those people which I would always helplessly, and in whatever bitterness of spirit, love above all else." Love. Notice how this word, which could have popped up so much earlier in the sentence, keeps getting pressed beneath the surface until the very end, so that by the time it does emerge it comes up like a revelation that the reader already knew was there.
On the one hand, the idea of love is simply clarifying the main idea of the sentence: it is a reason why David wants to go back home. Yet, on the other, he here presents an entirely new idea: it is possible to love someone helplessly, against one's will. It's as if we can no more choose the ones we love than we can choose our height or the color of our eyes.
This sentence, like so many others in the book, unfolds. One idea rolls into another and then another, and perhaps they all add up to one single idea, big and beautiful and full of contradictions. Looking at the sentence, one realizes that the entire story unfolds in the same way. It's as if David has cleared out an excavation site for his past and he is digging, digging clause after clause, against his own repression, in an effort to see the truth.
We only use one example because James Baldwin is a veritable master of the sentence. You can find more ideas in one of Baldwin's sentences than you can find in some novels.