I found A Maze of Stars by John Brunner to be an intriguing and slightly unusual book. The book does not seem to read like a novel with a continuous story from beginning to end, so much as a series of short stories or vignettes. The story takes place in the Arm of Stars, the last section of our galaxy to be colonized by humanity. A vast sentient Ship was built to travel along the Arm of Stars, seeding each habitable planet with the colonists who chose to settle there. After its great mission was complete, the Ship was supposed to stand by and monitor the progress of the colonies and rescue any population that was in danger.
Such was the plan. Instead the Ship found itself to be cast back and forth through time, compelled to retrace its journey along the Arm of Stars again and again centuries after the settlement of the Arm, observing but not contacting like an interstellar Flying Dutchman. At the end of each journey, the Ship travels through time again, emerging at the first planet it visited at a seemingly random time. In A Maze of Stars, the Ship finds itself at the earliest of its voyages, only 500 years after it seeded the colonies. The Ship travels from planet to planet with some knowledge of each planet’s future history, though there is much that remains hidden from the Ship. Some of the colonies have been successful, with a few even beginning to build star ships of their own. Many more are surviving with difficulty and more than a few are failures, the colonists destroyed by the hostile conditions of the planet they colonised. The Ship can recognise that some apparent successes will falter and fail, while some failures will recover. Occasionally, the Ship is able to exploit a loophole in its programming and take along a passenger on its travels. By the time the Ship reaches the end of its path, it learns why it was built and why it is sentenced to retrace its path again and again.
There is one issue raised by A Maze of Stars that I have not seen anywhere else in science fiction or nonfictional speculations about colonizing extra-terrestrial worlds. No life form on Earth either as an individual or a species exists in isolation. Every type of plant or animal lives in a complex ecosystem, composed of not just the obvious predators and sources of food, but also on a microscopic level with the bacteria than live around us and within us. Every form of life on Earth is to some extent depended on a vast web of interactions that are not always well understood. What would happen if some organisms are removed from that web and transplanted to a world with its own native lifeforms? Would the newcomers compete with the native life. Would alien organisms be toxic to creatures from Earth? What about the ecosystem of bacteria that each of us carries around with us and helps with our digestion? Could we maintain the proper balance in an alien world? Scientists have also recently learned that it is possible for DNA to be transferred between different species by viruses, and that these transfers have helped the process of evolution along. What if human colonists pick up alien DNA? Will they be able to remain human. The attempts of the various colonists to protect themselves against and adjust to the alien environments they have found themselves in is an important backdrop to the story of each planet the Ship visits and is a major factor in the success or failure of each colony, and the questions raised about the future of human development make the book worth reading.