Does anyone know this book...woman waking up next to a stranger

I've been sat trawling through book reviews and recommendations the past couple of days with the idea of hopefully add some promising titles to my Christmas list...exciting!!

Yesterday, I came across a book and now I can't for the life of me remember the title and I obviously forgot to write it down! If anyone can help me out that would be absolutely fantastic as I am so frustrated at myself right now...here goes...

All I remember is that the book opens with a women waking up and not recognising the person sleeping next to her (she mentions the colour of the persons hair - blonde, I think) and she goes on to remember it was her husbands idea for them to sleep with other people. I think she gets up and leaves the room and acknowledges her husband on the landing...that is all I can recall. Please help someone!!