Who is Bob Cantor from Dead in the Water: A Novel and what is their importance?

Bob Cantor is an ex-cop that has often helped Stone on cases in the past. Stone calls Cantor when he becomes involved in Allison's case. Cantor is the one who learns about Manning's financial situation and about his divorce from Libby Manning. Cantor proves to be invaluable in the end when Stone begins to suspect that Forrester is not who he says he is, leading to Forrester's arrest and extradition to St. Marks.