Like Lost, the book opens with a mystery that never gets solved.
Like Lost, the book pretends to explore the mystery, while instead exploring the lives of the interrelated characters in detail.
And like Lost, in the end, the answer isn't the answer to the mystery, the answer is... love.
Which I understood when I watched it, but anghara articulated it: "They were all remembering - they were all remembering LOVE. And there was something utterly powerful in that, something visceral, something that I NEVER really expected to see explored in contemporary American television."
It clicked then.
The last bit of The Bridge of San Luis Rey says:
"But soon we shall die and all memory of those five will have left the earth, and we ourselves shall be loved for a while and forgotten. But the love will have been enough; all those impulses of love return to the love that made them. Even memory is not necessary for love. There is a land of the living and a land of the dead and the bridge is love, the only survival, the only meaning." (emphasis mine)
You almost have to wonder why Sawyer wasn't reading this book at one point. Only, it might've been too big a tip-off.