The final installment in the "Guild of Specialists" trilogy, Operation Storm City, pretty much has it all: Secret societies within secret societies. Clever codes using arcane symbols. Swordfights a mile in the air involving arcing bolts of electricity. Zeppelin sabotage. Vengeful Tsarists. Double-crosses inside ancient labyrinths. Prehistoric doomsday devices. Tattooed lips. Horse-mounted Cossack flamethrowers. (Yes, seriously, HORSE-MOUNTED COSSACK FLAMETHROWERS!)
In Britain (where the series was first published, and where the third book is already out), this trilogy has been called "The Da Vinci Code meets Alex Rider." If, like me, you're not familiar with the British Alex Rider books, you can think of it as The Da Vinci Code meets Johnny Quest meets Lara Croft meets Young Indiana Jones meets Sky Captain and the World of Tomorrow. The story, set in post-WWI India and China, follows teenage sister and brother Becca and Doug as they search for their lost parents--members of the mysterious Honorable Guild of Specialists--while racing against time and an eccentric rogues' gallery trying to track down elusive gyrolabe "gravity devices" and find the legendary Storm City of Ur-Can.
The cast is over-the-top colorful (from the salty "aviatrix" and oil-heiress Liberty da Vine to the crazed, Bolshevik-hating General Pugachev), and the action is fast-moving and cinematic, told in part through Becca's diary entries and Doug's pencil sketches. It's all the supporting material, though, that makes this series so unique, and the book is littered with diagrams, photographs, portraits, newspaper clippings, historical asides (many even factual!), and letters--not to mention many lushly detailed drawings, including several dense fold-outs.
The series has a wide recommended age range, from 9 all the way up to high school, but it's definitely more PG-13 than PG, with no bashfulness about death (or planetary apocalypse, for that matter). Operation Storm City comes out in the U.S. in May--which should give you plenty of time to catch up on the first two books in the series, Operation Red Jericho and Operation Typhoon Shore!
(cross-posted at Omnivoracious)