The first of probably four 1632 books set in 1634. Now what comes to your mind when you think of 1634? The trial of Galileo of course! OK, imagine you're a bunch of American Kids in Venice in 1634, with an enthusiastic (if small) local supporting and a vague knowledge of history; what do you do? Try and rescue poor old Galileo of course.
Never mind the fact that he wasn't in much danger in the original history, and "current" history is changing all the time and such like.
I think I can't really improve on that section of Starlog's review quoted on the back of this volume:
…a mixture of Machiavellian intrigue, old-fashioned romance, travelogue, and Keystone Kops comedy. There's plenty of meat on the bones of this book, and it's well-seasoned.