You don't use "un facile livre": 90% of French adjectives are placed after the noun.
To know more about it: http://french.about.com/od/grammar/a/adjectives_4.htm
I keep getting these questions in lesson 7, while these words are clearly vocabulary of lesson 1. The original vocabulary of lesson 7( it happens on every lesson > 1, btw ) was reduced to incroyable alone + all of lesson 1's vocabulary. If created a discussion under troubleshooting.