by Robert B. Parker (1982)
A "specialty whorehouse" in Providence is a key location in this detective novel featuring Spenser, Parker's iconic P.I., according to the summary on Goodreads:
Pretty teenager April Kyle is in grown-up-trouble, involved with people who'd beat her up for a dollar and kill her for five. Now she's disappeared, last seen in the Combat Zone, that side of Boston where nothing's proper, especially the sex for sale. With Hawk, his sidekick, Spenser takes on the whole X-rated industry. From a specialty whorehouse in Providence to stylish Back Bay bordellos, he pits muscle and wit against bullets and brawn until he finds what he's looking for: April Kyle, little girl lost.