This is the fourth installment in the companion series to The 39 Clues, a wildly popular multi-media mystery series for tweens. With short suspenseful chapters, Shatterproof will not disappoint fans of the previous series, but it is not necessary to have read the other books to enjoy this one. This series, however, is best read in order. The relationship between orphans Amy (sixteen) and Dan (thirteen) continues to be a close one, although their experiences hunting for the Clues and protecting their family have changed them both. Dan, still snarky and funny, is now also a bit darker and sometimes withdrawn. Amy, has all but left her confidence issues behind and continues to demonstrate leadership. The story involves lots of action and a bit of violence, as two minor characters are murdered. Set primarily in Berlin, Mumbai, and Timbuktu, the plot also offers some interesting lessons in history. Tween readers who love mystery and suspense--even reluctant ones--will enjoy this fourth book in the Cahills vs. Vespers series and will look forward to the next installment.