Harry Harrison - Deathworld

Jason dinAlt is a gambler whose psi talents enable him to influence the fall of the dice. He is contacted by Kerk, an ambassador from the planet Pyrrus. Kerk wants dinAlt to gamble and win with his money, to raise enough money for him to buy weapons for the Pyrrans to fight a war. They are fighting against their planet, where every plant and animal seems to have a way, and a wish, to kill men.

Intrigued by these circumstances, dinAlt travels back to Pyrrus with Kerk. Here he must learn to survive in the high gravity, and train his reflexes to deal with sudden attacks from the flora and fauna. dinAlt decides to find out why the planet is attacking the Pyrrans.