Excerpt from The Girl on the Train by Paula Hawkins, plus links to reviews, author biography & more

I blink hard and Tom's gone. We're still at the signal. I can see Jess in her garden, and behind her a man walking out of the house. He's carrying something—mugs of coffee, perhaps—and I look at him and realize that it isn't Jason. This man is taller, slender, darker. He's a family friend; he's her brother or Jason's brother. He bends down, placing the mugs on the metal table on their patio. He's a cousin from Australia, staying for a couple of weeks; he's Jason's oldest friend, best man at their wedding. Jess walks towards him, she puts her hands around his waist and she kisses him, long and deep. The train moves.

I can't believe it. I snatch air into my lungs and realize that I've been holding my breath. Why would she do that? Jason loves her, I can see it, they're happy. I can't believe she would do that to him, he doesn't deserve that. I feel a real sense of disappointment, I feel as though I have been cheated on. A familiar ache fills my chest. I have felt this way before. On a larger scale, to a more intense degree, of course, but I remember the quality of the pain. You don't forget it.

I found out the way everyone seems to find out these days: an electronic slip. Sometimes it's a text or a voice mail message; in my case it was an email, the modern-day lipstick on the collar. It was an accident, really, I wasn't snooping. I wasn't supposed to go near Tom's computer, because he was worried I would delete something important by mistake, or click on something I shouldn't and let in a virus or a Trojan or something. "Technology's not really your strong point, is it, Rach?" he said after the time I managed to delete all the contacts in his email address book by mistake. So I wasn't supposed to touch it. But I was actually doing a good thing, I was trying to make amends for being a bit miserable and difficult, I was planning a special fourth-anniversary getaway, a trip to remind us how we used to be. I wanted it to be a surprise, so I had to check his work schedule secretly, I had to look.

I wasn't snooping, I wasn't trying to catch him out or anything, I knew better than that. I didn't want to be one of those awful suspicious wives who go through their husband's pockets. Once, I answered his phone when he was in the shower and he got quite upset and accused me of not trusting him. I felt awful because he seemed so hurt.

I needed to look at his work schedule, and he'd left his laptop on, because he'd run out late for a meeting. It was the perfect opportunity, so I had a look at his calendar, noted down some dates. When I closed down the browser window with his calendar in it, there was his email account, logged in, laid bare. There was a message at the top from aboyd@cinnamon.com. I clicked. XXXXX. That was it, just a line of Xs. I thought it was spam at first, until I realized that they were kisses.

It was a reply to a message he'd sent a few hours before, just after seven, when I was still slumbering in our bed.

I fell asleep last night thinking of you, I was dreaming about kissing your mouth, your breasts, the inside of your thighs. I woke this morning with my head full of you, desperate to touch you. Don't expect me to be sane, I can't be, not with you.

I read through his messages: there were dozens, hidden in a folder entitled "Admin." I discovered that her name was Anna Boyd, and that my husband was in love with her. He told her so, often. He told her that he'd never felt like this before, that he couldn't wait to be with her, that it wouldn't be long until they could be together.

I don't have words to describe what I felt that day, but now, sitting on the train, I am furious, nails digging into my palms, tears stinging my eyes. I feel a flash of intense anger. I feel as though something has been taken away from me. How could she? How could Jess do this? What is wrong with her? Look at the life they have, look at how beautiful it is! I have never understood how people can blithely disregard the damage they do by following their hearts. Who was it said that following your heart is a good thing? It is pure egotism, a selfishness to conquer all. Hatred floods me. If I saw that woman now, if I saw Jess, I would spit in her face. I would scratch her eyes out.

Excerpted from The Girl on the Train by Paula Hawkins. Copyright © 1905 by Paula Hawkins. Excerpted by permission of Riverhead Books. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.