I no longer remember where I heard about this book, but it stuck in my mind as something I should read if I got the chance.
I spoke to a fellow aficionado with very specific tastes. Among other features of his personality, he really only considers so-called hard science fiction to be genuine science fiction; the rest is just fantasy with space ships.
By those standards, The Engines of God is not science fiction, but it is closer and I recommended it to him.
It is a science fiction (by my lights) novel, but centered around discovery and investigation and despite having one action sequence with ‘blasters’ (here called pulsers), a sequence that is gritty, unnerving, and realistic in feel, it resolutely not an action book. There are alien ruins but the aliens are long gone.
The highest praise I can give is that it makes a semi-sentient, apocalyptic space cloud seem realistic and explainable.