UnLunDun by China Mieville
This book is surprisingly not well-known, but it's an amazing book with enough action and weirdness packed in it for an entire series.
UnLondon is the shadow of the city it is named for, one of many abcities that collect the mildly obsolete detritus left behind by the citizens of London. UnLondon, and other cities like it, are places where the broken and neglected discover new purpose. It is a world in danger. The Smog threatens to choke the life out of every living thing it can find, to feed itself and drown the world in darkness.
Enter Zanna and Deeba, two ordinary girls who find themselves in UnLondon after a string of events lures them there. Zanna is the abcity's chosen one, the Shwazzy, who will ultimately destroy the Smog and save UnLondon. Unfortunately, the Smog knows this as well as the good guys, and injures Zanna before she can fulfill her destiny. Deeba, the Funny Sidekick, takes on the quest meant for Zanna. Helped by an abandoned milk carton and talking book of prophecy, Deeba must save the abcity and get back to regular London before everybody back at home forgets she even existed.
Now before you write this off as too weird or confusing, just hear me out. I read this book when I was like 12, and I've been reading it a few times a year since then and I still catch new stuff every time. It has quirky characters and monsters that tumblr will absolutely love (an old fashioned diver suit filled with fish named Skool, a man who quite literally has a pin head, ghosts, man-eating giraffes, a villain named Smog, and funny little things called "Black Windows"), and more puns than you could possibly count. I got my first copy for free, but when I bought it for a friend it was only like 12 USD.
Just read UnLunDun, it's totally worth it.