Recommendations for novels written in colloquial Chinese
You said "I need a book that's easy to find in the United States."

I don't know what kind of book is easy to find in the US.Let me give you some suggestions.

As for mystery, suspense, or thriller：


  蔡骏的小说：荒村公寓，荒村归来，天机，猫眼等等


I like novels written by 蔡骏。

As for Colloquial, full of modern, or even slangy, words and phrases：


  当年明月：明朝那些事


明朝那些事 is a history book of Ming dynasty.It's very easy to understand and full of Chinese wisdom.

I don't know whether you can buy these books in the US or not.If you don't mind the issues of copyright,all of these books are easy to download from the Internet.Especially 明朝那些事，It comes from a personal blog -----当年明月的博客
。

In fact, the best way to learn a foreign language is to involve in the culture.All novels are written in the past time,they can't provide you the most fashioned words and phrases in the other culture.If you can't live in China ,you should try to communicate with Chinese through the Internet.Registering in the weibo.cn(twitter),renren.com(facebook) is a good idea.