The Reassembled Man by Herbert D. Kastle.
I'm sure I have a copy somewhere because I remember the story but I cannot find it in the chaos that is my book collection. So here is a summary I found on the web:
Our “hero” is Ed Berner, a 38 year-old sap who has been beaten down by life. Married to a shrew, the father of two prepubescent kids, Ed makes a moderate living as a copywriter at a Manhattan ad agency.
The aliens are the Druggish.
The aliens (who initially speak in beatnik, having assumed it was the standard Earth language from their research) inform Ed that he’s been chosen to be their “recorder” of life on Earth. They wanted an average guy living an average life, and Ed fit the bill. In exchange they will grant him his wishes; anything Ed has previously longed for will be given him.
You don't say what you mean by old, but the book was published in 1964.