James S. A. Corey - Abaddon's Gate. Book three of the expanse

In James S. A. Corey's Abaddon's Gate, the third book of The Expanse series, humanity confronts its greatest frontier yet: an alien gate in orbit beyond Uranus. After its work on Venus, the protomolecule has constructed a massive ring leading into a "starless dark," a gateway to countless new worlds. Jim Holden and the crew of the Rocinante are among a vast flotilla of military and scientific vessels sent to investigate this mysterious artifact. However, the greatest threat comes not from the alien structure, but from the human factions vying for control and their own internal conflicts. Earth, Mars, and the Outer Planets Alliance all converge, bringing their political rivalries with them into the strange new environment the gate creates. As a vengeful plot against Holden unfolds, he and his crew become caught in the middle of a conflict that could spiral into a system-wide war. The novel explores themes of exploration, fear, and human nature as the characters must decide if the gate is a new beginning or an impending disaster.