While he was there , Cat 's Cradle became a best-seller , and he began Slaughterhouse-Five . That book is now called one of the best American novels of the 20th century . While he was there , Cat 's Cradle became a best-seller , and he began Slaughterhouse-Five , now considered one of the best American novels of the 20th century , appearing on the 100 best lists of Time magazine and the Modern Library .