Original: 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.
Simplified: 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.