The Summons, by Peter Lovesey

In the first Peter Diamond mystery, The Last Detective,
Diamond ends the book in grand style by resigning from the Bath CID. He
spends a fair amount of time unemployed and underemployed until this book,
when the Bath CID needs him again. It seems that a guy Diamond put away for
murder has escaped from prison and kidnapped the Chief Constable’s
daughter, and the only one he’ll talk to is Diamond himself. He claims
to be innocent of the murder, and he wants Diamond to prove it. The CID
is interested only in capturing the guy before the Chief Constable’s
daughter is hurt, and they want Diamond to sweet-talk him; they have no
intention of re-opening the case.

But Diamond’s an honest man; that’s why the escapee is willing to work
with him. And though it seemed like an open-and-shut case at the time,
and still seems like one now, if Diamond says he’ll look into it, look
into it he will, whatever the top brass say. And as he’s still a
civilian, they can’t stop him….

The average Lovesey novel has some delightful twists, turns, and
surprises, and this one is no exception; though, honestly, I’d kind of
like to see a novel in which Peter Diamond isn’t going it mostly
alone.