Leviathan, Behemoth, and Goliath, by Scott Westerfeld.
Leviathan opens in an alternate late-July 1914, with Prince Aleksandr, son of Archduke Franz Ferdinand of Austria-Hungary, being spirited away by his fencing tutor and the master of mechaniks late at night. They climb into their Stormwalker and head for Switzerland. Anyone passingly familiar with real-world history may recall that Franz Ferdinand's assassination in late July 1914 was the spark in the tinderbox that led to the First World War, and if you made that connection, you would be correct.
Up in England, a girl named Deryn Sharp is taking the midshipman's exam for the British Navy (Air Navy? I forget what he called it, exactly.) The thing is, only boys are allowed to be middies, because it's 1914 and the military is a bit sexist like that. So Deryn calls herself Dylan, cuts her hair short, and dresses like a boy. Because of a storm during the first test flight using a Huxley (a jellyfish-hot air balloon), she ends up on the great airbeast Leviathan, a zeppelin-whale.)
The alternate part of Westerfeld's 1914 is that the world is divided between Darwinists and Clankers. The Darwinists have figured out how to take the "life strands" of creatures and fiddle with them in the lab, thus creating living airships and beast weapons, like strafing hawks and bats who eat razor blades stuck inside fruit then poop them onto their target. The living airships are an ecosystem, and it requires a delicate balance.
Clankers use mechaniks, or diesel-powered robots of a sort. The Stormwalker moves on legs, and the land ships are sort of like tanks on legs. Pretty straightforward.
Except, because the world in 1914 was pretty complicated, Alek's escape isn't so easy. They have enemies not just in the Darwinists, but among their own theoretical allies, the fellow Clankers in Germany. Deryn is always in danger of being discovered as a girl and kicked out of the service.
Through a series of events, Alek ends up on Leviathan with Deryn, and they become friends and have adventures while saving the world. (This book is targeted to a teenaged/young adult audience.) It's a great, fun read, and the story ends up taking them literally around the world, to Turkey (the Ottoman Empire), Russia, Japan, the US, and Mexico.
One thing I enjoyed were Westerfeld's endnotes on each book, explaining briefly what the real history was and how he changed it. I'm more than passingly familiar with the beginning of the Great War, and I enjoyed noticing things that I knew were the same (or close) and finding things he'd changed.
I purchased this for my kobo e-reader, and the only complaint I have is that it's hard to see the illustrations on the e-ink screen.
If you like adventures with mostly-happy endings, you'll enjoy this series.