Tuesday's Gone by Nicci French

Psychotherapist Frieda Klein finds herself working with the London police again, after a decomposing body is found in the apartment of a woman named Michelle Doyce. Michelle is mentally ill and cannot give the authorities any information about who the man is and how he died.  Frieda doesn't believe that Michelle is a murderer, neither does DCI Karlsson, but his bosses have closed the case, since Michelle is off the streets and in a psychiatric hospital.  Frieda, however, continues to pursue the matter, trying to figure out the identity of the dead man and his killer.  She also has nagging doubts that the killer she helped to apprehend in the series' first book, Blue Monday, is actually dead.  She believes that he's out there watching her....  I was anxiously awaiting the publication of Tuesday's Gone, since I loved Blue Monday.  While the first half of the book is first-rate, it lagged a bit in the second half for reasons about which I have to remain coy.  Definitely still worth a read, but reading the series in order is essential.