Series numeration from Fantastic Fiction.

"After returning home from a treacherous adventure, Stone Barrington is all too happy to settle back down in his New York City abode. But when he's introduced to a glamorous socialite with a staggering inheritance, Stone realizes his days are about to be anything but quiet. As it turns out, Stone's intriguing new companion has some surprisingly familiar ties and other far more sinister ones--including a nefarious enemy who gets too close for comfort. When it becomes clear that this miscreant will stop at nothing to get what he wants, and will endanger all whom Stone holds dear, Stone must step in to protect his friends and prevent a dangerous madman from wreaking havoc across the city." -- Amazon.com.