A kid walks into your bookstore and... Guess what? He's your son. The one you put up for adoption eighteen years ago. The one you never told anyone about. Surprise!   And a huge surprise it is.   It's a huge surprise to his adoptive mother, Monica, who thought she had a close relationship with Matthew, her nearly adult son. But apparently, he felt the need to secretly arrange a vacation to Cape Cod for the summer so he could meet his birth mother...without a word to either her or his dad.   It's also a surprise- to say the least-to Harlow, the woman who secretly placed her baby for adoption so many years ago. She's spent the years since then building a quiet life. She runs a bookstore with her grandfather, hangs out with her four younger siblings and is more or less happily single, though she can't help gravitating toward Grady Byrne, her old friend from high school. He's moved back to town, three-year-old daughter in tow, no wife in the picture. But she's always figured her life had to be child-free, so that complicates things.   When Matthew walks into Harlow's store, she faints. Monica panics. And all their assumptions-about what being a parent really means-explode. This summer will be full of more surprises as both their families are redefined...and as both women learn that for them, there's no limit to a mother's love.