I liked Ruth Ware's previous novel, The It Girl, pretty well. This one I liked even more, because the narrator/heroine, Jacintha ("Jack") Cross, is kind of a computer geek. ("You mean she's an IT girl?") Her story is chock full of nerdy goodness.
Jack and her husband, Gabe, do security consulting for companies concerned about bad-guy hackers creeping into their sensitive databases. They do "pen (penetration) testing", attempting white-hat breakins to their clients' infrastructure. Jack has mad physical intrusion skills: document forgery, lockpicking, impromptu social engineering, stealth, agility, etc. Gabe is a tech whiz, knowing common weak spots, turning them into garage doors for information.
It all starts when Jack breaks into the offices of their latest client, an insurance company. Gabe remains at their home office, abetting Jack's activities over the phone, guiding her to the company's server room.
It's a pretty good, suspenseful opening act. Jack is almost home free, until she gets nabbed, and hauled off to the police station. Some time is consumed until the company confirms that, yes, even as a hacker, she wears a white hat. But then things go really wrong when she returns home to find that Gabe's been brutally murdered.
And then things get even worse: the police consider her to be the prime suspect. Fortunately, she slips out of their clutches, vowing to find the actual culprits. And she does, but it takes some time, and a couple of very close calls. Think of Jack as a female-geek version of Richard Kimble in The Fugitive. There's even an analog of Gerard in DS Malik, who's obsessed with Jack's capture.
Looking back on my report on The It Girl, I mentioned that I could have done without the many, many descriptions of the protagonist's inner mental turmoil. That's a thing here too. And she undergoes a nasty injury on page 107 that (literally) festers throughout the book. I would have given up and gone to the emergency room around page 150 or so.
Jack overshares with the reader, Even to this extent (p. 175):
I winced as my bare skin made contact with the freezing toilet seat […]
Reader, I'm pretty sure this is an observation that Philip Marlowe, Elvis Cole, et. al. never made.
But it really is a very good, suspenseful mystery/thriller. There's also a very good twist near the end.