Published by Simon and Schuster, ISBN 9780857070005 
A huge airbeast was emerging from the gray clouds behind her, its reflective silver topside glistening in the sunlight.
The thing was gigantic - larger than St. Paul's Cathedral, longer than the oceangoing dreadnought Orion that she'd seen in the Thames the week before. The shining cylinder was shaped like a zeppelin, but the flanks pulsed with the motion of its cilia, and the air around it swarmed with symbiotic bats and birds.
Normally I don't read children's literature, I don't particularly like steampunk, and alternate history usually sets my teeth on edge. But I found myself enjoying this completely ridiculous yarn involving Alek, the only child of Archduke Ferdinand, stomping through Austria in a diesel-powered mech while the run from the people who assassinated his parents.
Even better, the other sub-plot involves Deryn, a Scottish lass who disguises herself as a boy to join the British Air Service, eventually ending up serving on the eponymous flying whalethe British have advanced genetic engineering thanks to Darwin, of course.. Soon war breaks out, with the mechanically minded Germany and Austria on one side and the biological based Britan and her allies on the other. Lot's of daring-do ensues.
This book probably counts as "Young Adult" but the target audience is obviously on the younger end of that meaningless tag. The characters are described as teenagers but act more like 10 year olds, which is where I think the book is most suited for. Despite going into some detail around the (somewhat fictionalized) political machinations in 1910s Europe, the book is pretty much none-stop g-rated actionNo deaths, little blood, lots of made-up swear words. Deryn's adventures in the Air Service are particularly well done, with lots of well written jumping off the sides of things into empty air, Zeppelin attacks, and general aerial mettle-proving.
I 
One of the many great illustrations that grace the pages of Leviathanimagine I would have loved this book as a pre-teen, but adult Andrew found it a little tiresome after the delight of tiny world-building details had worn thin. The plot is just a little predictable, any guesses you have based on my brief description will probably be correct. Leviathan also has one of those annoying endings that isn't a resolution but isn't really a cliff-hanger either. It just sort-of stops after setting up the sequel.
I did really like the well-realized world and the author's ear for dialog, Leviathan might be a kids book but it is a good kids book. The illustrations are pretty neat as well, just don't expect deep character development or an examination of the human condition. I would recommend this book to any child, but even young teenagers will probably find it babyish.