7/6/11 ** If you're ready for some action/adventure mixed with pseudo-science and historical fiction, grab this series. If this sounds like an odd mix, well, author Mowll pulls it off well.
Teen siblings Becca and Doug only want to know where their parents are. The last they heard, their folks had started a journey into the heart of 1920s Sinkiang desert - they were never heard from again. Now, more than a year later, both kids have been kicked out of school and have lived with at least 2 different relatives. They've been sent to live with their uncle on the research ship Expedient. Once there, they discover that their parents were part of a secret organization dedicated to preserving the mystery of extremely powerful machines called gyrolabes (at this point, we leave the realm of historical fiction and move into speculative fiction, ala Pullman's Dark Materials books.)
This is a YA novel with lots of extras: fold-out maps, schematic diagrams of ships & novel inventions, 1920s era photos (or at least those photo-shopped to appear to be from the 20s!), and copies of Becca's journal and Doug's sketchbook.
While the length and story complexity make this appropriate for most kids at 6th grade and up, there is very little 'objectionable' content in it, so I'd certainly offer it to my advance 4th graders, or even younger kids. Also, with male and female co-main characters, this would appeal to either gender.
I reduced the rating slightly because there were a few little plot inconsistencies and a few instances where the author 'fell out' of the time period, using phrases like "tires on a car" or "airplane." (I felt that automobile and aeroplane might be more era-appropriate.)