Rainbow Mars is a novelization of a series of short stories by Niven about time travel. The main character, Svetz, is charged by the government to go back in time to retrieve long-extinct animals for the infantile Secretary-General. But when Waldemar Ten is replaced by Waldemar Eleven, who wants planets and stars, Svetz must team up with Miya, a cosmonaut, to bring back Martians. What they find when they land on Mars is a world whose water is slowly being sucked up by the Hangtree, a tree so large its end lays in geosynchronous orbit with the planet. War has broken out, and the Martians are trying to survive. Svetz's and Miya's actions lead to the tree's implantation on Earth, and it is up to them to save their planet's future from the depths of its past.
Niven is my favorite science fiction writer, easily. A math professor, all his writing is based in science and is all physically possible, grounding the stories. Additionally, Niven has a great sense of humor, and it is manifested in each of his main characters. Beowulf Shaeffer is another great example, a man who, like Svetz, is unwillingly made into an explorer due to his inherent virtues. In Rainbow Mars, character development is a little thin, possibly due to the previously written short stories. All in all, though, another great read from Larry Niven.