A good thriller :-)

Some time ago I received this book as a gift in the book shop, for the Month of thriller-books 2005, and only now I've finally read it :-)

How can a man put himself in such trouble, for me that's nearly unimaginable. Two couples who seemingly have everything their heart's desires, who should be happy with what they have and can afford to do. And yet they act strangely, they cheat on one another, they are unhappy, they want to get out, do things differently. Anything to make it different than it is now.
And that ultimately culminates in murder.

What I did not understand in the text on the back, is the mention of the other secret that Sylvia is hiding. Maybe I overlooked it. In a while I'll read again, maybe I'll see it then. ( )