I just checked the Audible books page on Amazon.com. I clicked on the bestsellers, clicked on any of the books, and saw that it's $0.00 with Audible if you take a one-month trial, and get another credit on top, for a total of two books with the trial.
I just took advantage of the two-month trial with $10 Amazon GC a few months ago. And now this. Excellent.
YMMV. Free Bonus book appears to work for any book, not just the bestsellers.