Dragon of Black Glass by James Rollins

The third and penultimate book in the New York Times bestselling Moonfall series from thriller-master James Rollins, A Dragon of Black Glass is a tale of relentless adventure and the struggle for survival in a harsh world that hangs by a thread. What will they risk to save the world? With the apocalyptic threat of moonfall looming ever closer, Nyx and her allies must venture into the eternally sunblasted lands to search for an ancient weapon buried untold millennia ago. All the while, enemies close upon her flanks, and a greater danger lurks ahead. For beneath a desert turned to glass, hidden from the scorching heat, life thrives-both wondrous and monstrous. But a more fearsome menace lies even deeper, where an ancient army has been seeded to protect a secret from any who dare seek it out.