Just finished reading, and have to say it is quite excellent. The first two books followed the plot of the game, and while they fleshed out the background of characters, and filled in some details players may have missed, their appeal to me lay mostly in their familiarity, in the reliving of fond memories. (Except, it seems, I took the non-canon ending … though did I!?).
The Flood (and the upcoming fourth volume) break free from the constraints of the game, and go even deeper into the history and inner workings of the Nebula. And it’s quite surprising. Especially since there are two intertwined story lines where surely the outcome of the one must lead to the beginning of the other, but how this will come to pass is yet a mystery.
Well worth reading, but either play Heaven’s Vault beforehand, or never ever. And I don’t recommend latter.
And while I’m at it, this one’s been too good not to mention, but sadly it’s available in German only: Omni by Andreas Brandhorst. It’s a space opera paying homage to Star Wars and Ursula K. Le Guin, and while I cannot comment on Le Guin, it captures the spirit of the original trilogy quite nicely. What makes it stand out, however, is the plot. In a boring book, the protagonist marches from victory to victory and makes it seem effortless. In a gripping one, they overcome challenges by the skin of their teeth. In Omni, they suffer setback after setback, each worse than the previous and yet, somehow, they still make it through alive and triumphant.
And lastly, as honorable mention, Far from the Light of Heaven by Tade Thompson, a murder mystery on a colony ship en route to a distant planet. This one is more hard Sci-Fi, and perhaps not so much unlike Between Horizons in principle, but with its own unique take on the setting.