One Step Too Far by Tina Seskis
I read this self-published novel on my Kindle. Came across it after hearing the author speak at the Winchester Writers' Conference. She was there along with representatives from Amazon's KDP, as an example of a self-publishing success story.
I'm not surprised this book has been successful - it's a damn good read. Like many SP novels it could do with a better copy-edit but the mistakes aren't too distracting. The book keeps you turning the pages and wanting to know everyone's secrets.
One day, Emily leaves her family and gets on a train. Along the way she changes her identity completely and becomes Cat, and she never goes back, creating a new and very different life for herself. What triggered her to do this - that's the central question and the one which isn't answered until very near the end. You know it must be big, very big, but like many readers (judging by the reviews) I did not guess at all and when the twist/reveal came it was a shock. Cleverly done, and very satisfying.