No Good Deeds by Laura Lippman
Tess Monaghan is back. The story really starts when Tess' boyfriend Edgar "Crow" Ransome brings home a homeless teenager from a soup kitchen where he was delivering food. When Tess discovers that the boy, Lloyd, has information about the murder of a DA, she convinces him to talk to some reports, upon the promise of anonymity.
Of course, this causes quite a stir, and a pair of federal agents try to put pressure on Tess to reveal the boy's name. Frightened for Lloyd's safety, Crow takes him to a safe house, but that only serves to increase the desperation of the feds.
I was a bit surprised to learn the identity of the "bad guys:"; then I was surprised that I was surprised.
Great characterizations (even of the bad guys), great book. I had trouble putting it down, and even regretted getting to the end.