Painted Ladies by Robert B. Parker

Painted Ladies (Spenser Mysteries, No. 39) - A valuable painting has been stolen from a local museum. Spenser is asked to protect Dr. Ashton Prince during the exchange of ransom for the painting. Dr. Prince is murdered during the exchange, and the painting is destroyed.  Spenser being Spenser feels responsible and begins his investigation.  Apparently the painting was stolen originally by the Nazis during WWII and Prince had a connection to the family it was stolen from.