You can get The Girl on the Train for £7 in Asda, which annoyed me as I paid £12 for it in Waterstones, and it's very good.
I haven't read I Let You Go, but it sounds good so I hope you enjoy it.
I can second U2's recommendations of A Kind of Intimacy and The Book of You. I enjoyed both but I think AKOI is the better of the two.
True Things About Me by Deborah Kay Davies might be a good one for you as well. It's a bit different to the others but I remember enjoying it and wanting to know what happened.
Have you read Who Is Sarah Lawson by KJ Rabane? I think it's about £1 on Kindle at the moment and I really enjoyed it. There's a second book with some of the same characters as well, but I haven't read that one yet.
And the earlier Nicci French books are good if you haven't read them yet. The newer ones that are part of a series are good as well, but the stand alone ones are my favourites.
Also The Magpies by Mark Edwards is good if you haven't already read it, but I wouldn't bother with What You Wish For, it's weird and a bit disappointing.