The book starts off well, with nice characterisation, believable dialogue and a menacing conspiracy that does what any good (fictional) conspiracy should do - make the reader feel like no-one is safe.
There are thrills and chills and twists and turns a-plenty, and during all that time I was reading it, I was enjoying myself immensely...
...until the final act ruined everything.
Quite worryingly quickly, the story takes an abrupt and unbelievable turn from conspiracy thriller into sci-fi (and even though the sci-fi undercurrent was - admittedly - introduced somewhat early on, when it fully rears its out-of-place head it feels entirely abrupt, entirely ridiculous, and a massive cheat). And after that, when the conspiracy's reasons for being are unravelled, the level of ludicrousness becomes too much to swallow.
"Relentless" is the most initially promising, but ultimately massively disappointing novel I've yet read in my time.
(If only 2/3 of a good book were enough...)