The Space Eater by David Langford
Ken Jacklin is a combat soldier in the future. In order to decrease soldiers' tendency to survival instinct reactions in situations with a possibility of death, combat training involves numerous practice sessions in which the soldier sustains what could be fatal wounds. The soldiers are put into regeneration tanks to restore lost body parts, and then sent back for more combat practice. By "dying" multiple times, the soldier learns not to take dying so seriously, and therefore reacts to possible death based on military training rather than animal instinct.
Jacklin is selected for a very secret project. Years ago, a method referred to as Matter Transmission (MT), was used to send a colony to a planet 162 light years from Earth. The portal was lost. Later, scientists concluded MT caused dangerous stresses throughout space-time, and was responsible for some stellar novas that had occurred at that time. MT work was ended on Earth. Recently, there has been evidence of MT activity, presumably on the other planet. Earth believes they must get the other planet to stop all MT activity.
Jacklin is teamed with Rossa. Earth scientists design an MT portal with an opening smaller than an inch. Such small MT portals are believed to be less dangerous. Equipment is transported through the portal that constructs a structure in space in the other star system. Then Jacklin and Rossa are dissected, sent through the port in little pieces and re-grown in tanks in the structure. They are then to contact the other planet and claim they have arrived in an FTL ship. They are to try to convince the other planet to end MT work. If they are unable to convince the other planet, they are instructed to give them plans for a small opening MT device that would be less dangerous. Jacklin and Rossa speculate that Earth has other contingency plans they have not be informed about.
However, when they arrive near the planet, it turns out there is a big war going on between the original settlements on islands and settlements on a continent demanding political independence. The continent has vast reserves of petroleum, making it economically crucial. This makes the islanders unwilling to give independence. There's a nasty war with AI-controlled missiles. However, there is a treaty forbidding nuclear weapons and the like.
Jacklin and Rossa have been brought down to the island territories. The island government considers MT research essential to finding a weapon permitted by the treaty that could win the war for them - so the islanders are unwilling to stop MT activities. They might consider ending MT work if Jacklin and Rossa could provide them with some other technology to win the war - such as FTL tech. Jacklin and Rossa say they don't have that info. But the islanders are willing to try to torture information out of them.
The book explores the military mentalities, behaviors and methods. The ruthless logic makes efforts to address the MT danger nearly impossible. Even when it seems the war has ended, it's not that simple. This isn't military SF in the sense the war is in the background and never seen first hand by the characters. Rather it's more about the military thinking, military science work, getting information from captives and the like.
There is a significant part of the book about or revolving around speculative technology. This could be fascinating to many readers, although some readers may find some puzzling aspects of the technology. (See science notes below if you wish to consider such issues.)
And SF readers will find Earth’s contingency plan thrilling.
The mission is related to what they refer to as Matter Transmission. However, the method of transport involved can have different sized openings which will only allow objects that will fit through an opening of that size. Therefore, it doesn't seem objects are being "transmitted" in the sense of radio waves, laser beams or other energy signals. Also, when something is "matter transmitted" 100 light years, it doesn't take 100 years or more to get there, so neither energy transmissions or travel through normal space seems applicable. Since the method involves openings of various sizes, it circumvents the speed of light and they don’t have real FTL tech, it sounds like using a wormhole - although such things are never mentioned. The story's plot is based on the premise that "matter transmission", especially when larger openings are used, causes instabilities in space-time, leading to stellar novas. I'm not familiar with that being considered a risk with wormholes.
Another point is Rossa. She has been selected for the mission because of an ability she has. She can transmit messages in a method which is more or less a psychic power. It’s not telepathy in the normal sense. She can prick herself and individuals capable of “receiving” will feel the pain. By pricking herself in a way to convey Morse Code, the receiving person can de-code a meaningful message. The fact this is a limited sort of communication may make it less implausible than other psychic powers. On the other hand, the book seems to indicate she can send these messages instantaneously over interstellar distances, making it less plausible.