The Hours of the Virgin — Loren D. Estleman

Detroit PI Amos Walker searches for a priceless medieval illuminated manuscript -- and for evidence that can put his former partner's killer behind bars

Hired by a curator at the Detroit Institute of Arts to serve as his bodyguard during a transaction involving a stolen illuminated manuscript, Amos Walker enters a darkened skin-flick theater where the exchange is supposed to take place. When the deal goes south, he's lucky to leave with his life . . . and a new lead to pursue in collaring the man who murdered his partner 20 years ago.
 
In a case that features a wheelchair-bound pornographer and rare book collector, an ultra-slick art expert, a trophy wife, and a white-collar criminal, Walker faces one of the greatest challenges of his career as a present-day crime draws him back to one of the darkest episodes of his past.
 
The Hours of the Virgin is the 13th book in the Amos Walker Mysteries, but you may enjoy reading the series in any order.