Watching You, by Michael Robotham

Psychologist Joe O'Loughlin sees Marnie Logan twice a week to help her deal with the emotional and financial wreckage caused by her husband's disappearance.  When Daniel disappeared, he owed a loan-shark 30,000 £, and Marnie is expected to pay it back.  With two kids to support,  no access to any of Daniel's bank accounts, and a thug driving Marnie to her "escort" appointments and collecting on her debt, Marnie is in dire straits.  Joe recruits retired detective Vincent Ruiz to help him locate Daniel and get the loan shark off Marnie's back.  But the more they find out about Daniel's last acts, the more questions they have about Marnie.