Straight by the Rules Book Three in the Lilith Straight Series

Although temptress Lilith Straight didn't forge her contract with the Devil, she's obligated to obey it. But upon hearing the dastardly details of her latest assignment, Lilith vows to wriggle out from under the Devil's thumb one way or another. Unfortunately, Hell is in chaos, her bff is AWOL, and her eleven-year-old niece's fascination with the supernatural is tempting her to explore the otherworld. If only William Darcy, the hottest demon in Hell, would put the brakes on his own ego and give her a hand, Lilith might stand a chance.