Into the Gauntlet by Margaret Peterson Haddix

This book concludes the 39 Clues series. Can Dan and Amy reunite the family and discover the secret to the serum? In this book, there are great references to William Shakespeare as the kids learn who they can trust in their family. I wouldn't say that this series is great literature, but it is a good story, and I'm hoping that the hints at the end that seemed to point to the possibility of a second series are accurate!