Sorry for the vague description but this is what I can remember from it:
There is a couple and the woman is having an affair with someone - she goes to see him in a dirty old room above a shop I think. She finds out she has a brain tumour (I think its brain tumour - a serious illness anyway) but I don't think she tells her lover (or possible her husband). She ends up crashing her car through a wall and I think she dies. Her lover (or possibly husband) has found out about her being ill somehow - possibly by reading her post or by knowing the doctor at the hospital.
It could be a very rubbish book, I can't remember - but for some reason I have the story in my head and its annoying me! I thought it was One Day but thats a completely different story!