Cold Service by Robert B. Parker

Bad Business (Spenser) - While protecting Luther Gillespie, Hawk is shot in the back.  Gillespie and his whole family, except the boy in daycare, are murdered.  Spenser helps Hawk get his strength back, while investigating the murders.  He learns that the Ukrainian mob is responsible; he also learns that Tony Marcus’ son-in-law is involved somehow with that mob.  When Hawk returns to normal, he and Spenser exact their revenge.