Writin' on Empty: Parents Reveal the Upside, Downside, and Everything In Between When Children Leave the Nest
From the Introduction:

For eighteen years or so, ever since your baby's first cry, you are tuned in to her every move and mood: each tear, each skinned knee…right up to the first time behind the wheel, the first slammed door and the first date. Then, one day, the kids leave: some for college, some to don a uniform and join the service, some to follow a dream, and some to find space to grow. And now you are standing in the doorway of a vacant bedroom, feeling the silence and emptiness as it echoes through your home. You should be able to sleep easy, not waiting for that car to pull up, or that door to slam again. But maybe you can't. Maybe you discover that missing your child has turned into an ache that can't be touched. Or, perhaps your child has made a successful transition and you are already leafing through design books or travel brochures and planning the rest of your life. Maybe it was easier than you thought it was going to be. I handled that pretty well, you think...and then you get a phone call that begins with a catch in the throat: "Mommy..."