Yes, this book is, indeed, relentless. It is one of those stories best read at a single sitting, sweeping you along in scene after continuous scene of chases, villains, police, kidnappers, gunshots, double- and triple-cross, conspiracy and left-luggage lockers.
Tom Mahon is, at the start of the book, a happily married man with a lovely wife, two small children, a boring but well-paid job and a nice house on an upmarket estate (not a gated community but not far off). His life is suddenly turned upside down when, one weekend, he receives a phone call from an old friend, and hears the friend apparently being killed while shouting out Tom's address, presumably to his attackers.
Panicking, Tom packs his children into the car, drives to his mother-in-law, drops them there and charges off to the university where his wife is working overtime to warn her to stay away from home and danger. She isn't there, but a knife-wielding villain is. Things go rapidly downhill for poor Tom from this point. He knows the Balaclava-wearing villain has stabbed someone to death, but is it his wife? In struggling to escape, the police pick him up and believe him responsible for the crime against the unknown victim. Tom manages to persuade the cops to release him, but soon wishes he hadn't, as piece-by-piece his family life falls apart, his house is ransacked, his children are kidnapped and he is about to be tortured to reveal information he doesn't know.
RELENTLESS is an adrenalin-filled ride. Simon Kernick keeps a lot of balls juggling in the air, and rarely lets one drop. The downside to all the activity is that the characterisation is poor. The main detective, Bolt, is sympathetic and slightly more three-dimensional than everyone else. Another character, Tina, who provides some of the plot drive, has featured in previous books by this author, is also a more rounded portrait, but doesn't appear very much.
But the bulk of this book follows a sure formula: constant shifts of loyalty and endlessly changing upper hands follow on from each other so quickly that you can't keep up. People aren't what they seem, switch sides or reveal dark aspects of themselves. Mystery tough guys crop up and are inclined to answer Tom's increasingly bewildered questions by replying that they are "The NCS. National Crime Squad. Specifically, a specialist undercover team called The Guardians. You won't find us listed anywhere on the website. Our work's a complete secret." If you, like Tom, are happy to go along with this kind of stuff, you'll enjoy the book, until one side-change too many at the end gives one a slight sense of "so what?". But if you are looking for a couple of hours of escapist and easy reading, and are happy to suspend disbelief, this is the book for you. It isn't Lee Child, but it is racy and pacy.