From audible.com:
A heist by the Serenity crew goes badly wrong in a captivating original Firefly tie-in novel from the award-winning series by Titan Books.
City of sin
Neapolis: a desert city on planet Bethel where all manner of entertainment is found: High-stakes gambling, luxurious hotels, exclusive clubs, and any form of diversion imaginable may be had for a price. It’s the eve of the annual carnival: three days of decadent revelry, and Serenity arrives to take a security job, guarding a costly shipment.
An unattainable ransom
Tragedy strikes: The shipment is stolen, and the wealthy owner kidnaps Zoë and Book, holding them to ransom for the lost shipment’s value. If Mal can’t find the enormous sum of 500 platinum by the next evening, both of them will be killed.
A race against time
As the carnival begins, the crew must attempt the impossible, calling on contacts, calling in favors, and revealing hidden talents to save their crewmates’ lives. Meanwhile, the hostages have their own plans....