Blizzard of the Blue Moon (Magic Tree House, Book 36) (by Mary Pope Osborne)

This, the 36th book in Mary Osborne's wildly popular Magic Tree House, continues a sort of "Merlin Mission" sub-series. Kids won't need to have read any of the previous books to fall into this one. Previous books in the series had more realistic settings -- despite the magic portal that got them there. But the Merlin Mission books have added elements of fantasy to help the history lessons go down easy.

Osborne is a pro, and children won't know -- or mind -- that they're getting an education as they dive into the somewhat thin but generally entertaining plot. It's not great literature, but for kids hungry to graduate to chapter books (and for the parents who are hungry to see them move ahead, too) this book offers plenty of a low-key adventure. A couple of bad guys show up to keep things interesting, but the action never rises above a G-rated level.