Chop Line — Henry V. O'Neil

Things are looking grim for the human Alliance against the Sims. A revolution on Celestia is draining the war zone of badly needed troops, and Alliance Chairwoman Reena Mortas is being blamed for the debacle. Her husband and predecessor, Chairman Olech Mortas, remains missing in the faster-than-light mode of travel known as the Step.

Jander and Ayliss Mortas are putting their lives on the line against the Sims, he with the hardcore Orphan Brigade, and she with the elite, all-female Banshees. But when the mysterious alien shapeshifter from Jander's past reappears, he is sent on a secret mission that could possibly allow humanity to finally communicate with the Sims -- and end the war. 

Meanwhile, Reena's attempts to find her missing husband are yielding surprising revelations from inside the Step that could answer two of humanity's greatest questions. Who gave us the Step? What is making the Sims? In the penultimate book of the Sim War series, the truth of these hidden space entities begins to emerge.