The Copernicus Legacy #2: The Serpent's Curse by Tony Abbott

Wade and his friends have collected the first relic and are now searching for the second. They're trying to keep the Teutonic Order from getting all twelve relics; the twelve relics together will create a time machine designed by Nicholas Copernicus in the 1500's. The Order has captured Wade's stepmother and plans to send her back in time. The kids must decode Copernicus's 500-year-old clues, as they search throughout Russia and Italy. The Order has spies all over the world, their leader is ruthless, and she has sent an assassin after the kids. And Wade's mom has less than two days before she disappears into the past. She'll be stuck in the year 1517 forever, if she survives the trip back in time.

You should read book one before reading this one. As with the first book in the series, the time machine offers an element of fantasy, but this book reads like an adventure story. The clues left by Copernicus provide mystery to the plot, as the characters try to decode messages and locate the relics. The Order creates suspense, as it tries to find the relics first and tries to kill or capture the main characters. The plot hints that there might be something special about one of the kids, as the evil woman orders the death of all of the main characters except for "the one". The fantasy, mystery, and adventure are sure to interest most middle grade readers.